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git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
Pull SMP core updates from Thomas Gleixner:
- Reduce the preemption disabled sections in smp_call_function*().
The various smp call functions keep preemption disabled accross the
full operation which includes the wait for completion. Especially the
latter can take some time when one of the target CPUs is not
immediately responding to the IPI, which can result in large latency
spikes.
To improve this provide a per task CPU mask to track the CPUs to wait
for. That makes the information required for the wait task local and
therefore allows to reenable preemption before the wait.
While this comes with moderate extra memory cost this reduces SMP
function call induced latency measured in a fleet for high priority
tasks from ~17ms to ~1.5ms (~90%).
- Reduce the overhead of the CSD debug code by replacing the heavy
memory barriers with smp_store_release()/acquire()
- Remove obsolute unused hotplug states
* tag 'smp-core-2026-08-17' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
scftorture: Remove preempt_disable() in scftorture_invoke_one()
smp: Remove preempt_disable() from on_each_cpu_cond_mask()
smp: Remove preempt_disable() from smp_call_function()
smp: Enable preemption early in smp_call_function_many_cond()
smp: Alloc percpu csd data in smpcfd_prepare_cpu() only once
smp: Use task-local IPI cpumask in smp_call_function_many_cond()
smp: Refactor remote CPU selection in smp_call_function_any()
smp: Enable preemption early in smp_call_function_single()
smp: Disable preemption explicitly in __csd_lock_wait()
cpu/hotplug: Remove CPUHP_AP_ARM_CORESIGHT_CTI_STARTING
smp: Use release stores for csd_lock_record() state
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Commit b0473dcd4b1d ("smp: Improve smp_call_function_single()
CSD-lock diagnostics") changed smp_call_function_single() so that,
when CSD lock debugging is enabled, async !wait calls use the
destination CPU csd_data. That improves diagnostics, but it also removes
the single-writer property that made the old csd_lock() safe: multiple
CPUs can now prepare the same destination CPU CSD concurrently.
csd_lock() currently waits for CSD_FLAG_LOCK to clear and then sets the
bit with a non-atomic read-modify-write. Two senders can both see an
unlocked CSD, set the bit, overwrite the callback fields, and enqueue
the same llist node. Re-adding a node that is already the queue head can
make node->next point to itself, leaving the target CPU stuck walking
call_single_queue. Later synchronous work, such as a TLB shootdown, can
then remain queued and trigger soft-lockup warnings or panics.
Keep the single csd_lock() implementation, but when CSD lock debugging is
enabled, acquire CSD_FLAG_LOCK with try_cmpxchg_acquire(). This makes the
destination CPU CSD a real atomic lock in the only configuration where it
can be shared by multiple remote senders, while preserving the existing
non-debug fast path.
Fixes: b0473dcd4b1d ("smp: Improve smp_call_function_single() CSD-lock diagnostics")
Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20260716004539.13983-2-paulmck@kernel.org
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Commit b0473dcd4b1d ("smp: Improve smp_call_function_single()
CSD-lock diagnostics") made smp_call_function_single() use the destination
CPU's csd_data when CSD lock debugging is enabled. That lets the debug code
associate a stuck CSD lock with the target CPU, but it also means the CPU
argument is used in per_cpu_ptr() before generic_exec_single() has a chance
to validate it.
This becomes unsafe when smp_call_function_any() cannot find an online CPU
in the supplied mask. In that case the selected CPU can be nr_cpu_ids, and
the !wait path calls get_single_csd_data(cpu) before generic_exec_single()
returns -ENXIO. With csdlock_debug_enabled set, that indexes the per-CPU
offset array with an invalid CPU number.
Use the destination CPU's csd_data only when the CPU number is within
nr_cpu_ids. For invalid CPU numbers, fall back to the local CPU's csd_data
and let generic_exec_single() perform the existing validation and return
-ENXIO.
Fixes: b0473dcd4b1d ("smp: Improve smp_call_function_single() CSD-lock diagnostics")
Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Muchun Song <muchun.song@linux.dev>
Link: https://patch.msgid.link/20260716004539.13983-1-paulmck@kernel.org
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smp_call_function_many_cond() handles the preemption and CPU pinning
requirements internally. on_each_cpu_cond_mask() only builds the call
flags and forwards the request to that helper.
Remove the outer preempt_disable() and preempt_enable() pair from
on_each_cpu_cond_mask().
Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://patch.msgid.link/20260709122933.4021501-9-zhouchuyi@bytedance.com
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smp_call_function_many_cond() handles the preemption and CPU pinning
requirements internally. smp_call_function() only forwards the request to
that helper for cpu_online_mask and does not access CPU-local state on
its own.
Remove the outer preempt_disable() and preempt_enable() pair from
smp_call_function().
Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://patch.msgid.link/20260709122933.4021501-8-zhouchuyi@bytedance.com
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smp_call_function_many_cond() still has to keep the caller pinned to the
current CPU while the remote IPI request is built and dispatched. This
protects the queueing state and CPU-hotplug boundary that are required
before the synchronous wait starts:
- It protects the current CPU's per-CPU scratch cpumask,
cfd->cpumask_ipi. Another task running on the same CPU could otherwise
enter smp_call_function_many_cond() and reuse that scratch cpumask
before the current caller has finished building and sending the IPI
request.
- It provides the CPU-hotplug exclusion required by the CSD queueing
side. New CSDs must not be queued after smpcfd_dying_cpu() has flushed
the outgoing CPU's callback queue. Keeping preemption disabled until
all required CSDs have been queued and the corresponding IPIs have
been sent prevents CPU offline from crossing that boundary in the
middle of the queueing operation.
The CSD acquisition side also relies on that caller-side CPU pinning.
csd_lock() waits for CSD_FLAG_LOCK to clear and then marks the CSD busy
with a regular store, so another task on the same CPU must not be
allowed to acquire and reinitialize the same per-CPU CSD concurrently.
After the callbacks have been queued and the IPIs have been sent, the
caller only performs the final csd_lock_wait() completion wait. If it is
preempted there, another task running on the original CPU may enter
smp_call_function_many_cond(), but any attempt to reuse the same per-CPU
CSD will block in csd_lock() until the previous callback clears
CSD_FLAG_LOCK. The final csd_lock_wait() does not acquire or reinitialize
the CSD, so it does not need the same caller-side preemption-disabled
protection.
The wait mask is task-local, so it cannot be overwritten by another task
on the original CPU. The per-CPU CSD storage also remains allocated
across CPU offline, so csd_lock_wait() can safely dereference it even if
the target CPU is offlined after the caller is unpinned.
With those requirements satisfied, enable preemption before the
synchronous csd_lock_wait() loop. This makes the potentially long wait
preemptible and migratable while keeping the CPU-pinned section around
the remote CPU selection and IPI dispatch.
Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://patch.msgid.link/20260709122933.4021501-7-zhouchuyi@bytedance.com
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smp_call_function_many_cond() uses per-CPU CSD objects when queueing
callbacks to remote CPUs, and the wait path later dereferences those CSDs
from csd_lock_wait().
Making the wait path preemptible allows the initiating task to be
preempted or migrated before it waits for completion. A target CPU can be
offlined in that window. If smpcfd_dead_cpu() frees the target CPU's
per-CPU CSD storage, csd_lock_wait() can later dereference freed memory.
One way to protect the CSD storage is to free it via RCU or after a
synchronization step in the CPU offline path, but that would add
unnecessary complexity and can delay CPU shutdown.
Allocate the per-CPU CSD storage the first time a CPU comes up and keep
it allocated when the CPU is offlined. This allows csd_lock_wait() to
access the CSD even when the target CPU is offlined after preemption is
re-enabled and before the wait is invoked.
Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Muchun Song <muchun.song@linux.dev>
Link: https://patch.msgid.link/20260709122933.4021501-6-zhouchuyi@bytedance.com
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smp_call_function_many_cond() uses the per-CPU cfd->cpumask as the list
of remote CPUs to wait for. That is safe while the caller remains pinned
to the current CPU for the whole operation, because another task cannot
run on the same CPU and reuse the per-CPU mask.
The synchronous wait is the long-latency part of the operation. To make
that wait preemptible, the mask iterated by csd_lock_wait() must remain
stable even if the task is preempted or migrates. If the wait used the
per-CPU cfd->cpumask after dropping CPU pinning, another task scheduled
on the original CPU could enter smp_call_function_many_cond() and
overwrite the mask while the first task is still iterating it.
Give each task private IPI cpumask storage and use it as the wait mask in
smp_call_function_many_cond(). Other cpumask storage choices do not fit
this use case:
- Per-CPU storage is the state that becomes unsafe once the wait is
made preemptible. After the caller drops CPU pinning, another task
scheduled on the original CPU can enter smp_call_function_many_cond()
and reuse the same per-CPU mask.
- Stack storage is not suitable for large NR_CPUS or
CONFIG_CPUMASK_OFFSTACK=y configurations. The wait mask needs to
scale with cpumask_size(), and putting that storage on the stack is
not acceptable on large systems.
- Allocating the mask inside smp_call_function_many_cond() would put an
allocation and a failure path in the generic IPI path. A sleeping
allocation is not suitable because callers have historically only
provided a preempt-disabled context, not a sleepable one. GFP_ATOMIC
would avoid sleeping, but a failure fallback would make the latency
improvement opportunistic instead of guaranteed.
The users are not limited to a small, pre-identifiable class of tasks. On
x86, ordinary tasks can reach this path through TLB flushes during exit,
unmap and reclaim, so allocating the mask only for a known subset of
tasks is not straightforward.
The memory cost is explicit: one word is added to task_struct. When
cpumask_size() fits in that word, the mask is stored inline and no
separate allocation is needed. Larger systems allocate cpumask_size() per
task; on x86-64 NR_CPUS=8192 this is 1 KiB per task. For context,
x86 already carries several KiB of per-task architecture and FPU state,
depending on the enabled features and configuration. That does not make
the extra cpumask free, but it puts the large-NR_CPUS case in
perspective.
Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://patch.msgid.link/20260709122933.4021501-5-zhouchuyi@bytedance.com
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smp_call_function_any() disables preemption across the entire operation:
selecting a target CPU, enqueueing the IPI, and synchronously waiting for
the remote CPU. smp_call_function_single() already re-enables preemption
before the synchronous csd_lock_wait(), so callers of
smp_call_function_any() should benefit from the same shorter
preemption-disabled section.
Simply removing get_cpu() and put_cpu() from smp_call_function_any()
would leave the preemption disablement entirely to
smp_call_function_single(). That opens a preemption window between
selecting the remote CPU, for example via sched_numa_find_nth_cpu(), and
dispatching the IPI in smp_call_function_single(). If the selected CPU is
fully offlined in that window, smp_call_function_single() fails its
cpu_online() check and returns -ENXIO to the caller, violating the
guarantee that smp_call_function_any() executes on any online CPU in the
mask.
Move the remote CPU selection into a common
__smp_call_function_single() helper. Keep the target CPU selection and
IPI dispatch within the same preemption-disabled region, while still
allowing the wait path to use the shorter preemption-disabled section
provided by smp_call_function_single().
Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://patch.msgid.link/20260709122933.4021501-4-zhouchuyi@bytedance.com
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smp_call_function_single() disables preemption while it validates the
target CPU, prepares the call single data, queues the callback and sends
the IPI.
For the !wait case, preemption protects the per-CPU csd_data from
concurrent modification by another task on the same CPU. For the wait
case, the CSD is stack allocated and no other task can reuse it. CPU
pinning is still required until the callback has been queued and the IPI
has been sent, to ensure that the target CPU cannot be offlined after the
online check but before dispatch.
After generic_exec_single() has queued the callback, the synchronous
csd_lock_wait() invocation at the end of the execution does not require
the caller to remain pinned to the current CPU.
Enable preemption before csd_lock_wait() to shorten the
preemption-disabled section.
Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://patch.msgid.link/20260709122933.4021501-3-zhouchuyi@bytedance.com
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The CSD lock wait debugging code in __csd_lock_wait() must run with
preemption disabled. The smp function call mechanisms which invoke it
currently keep preemption disabled across the wait, so the debugging code
inherits that guarantee from its callers.
Keeping preemption disabled across the whole smp function call operation
can induce large scheduling latencies. Shortening the caller-side
preemption-disabled region would invoke __csd_lock_wait() with preemption
enabled.
Prepare for that by disabling preemption explicitly around the CSD lock
wait debugging code in __csd_lock_wait().
Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Muchun Song <muchun.song@linux.dev>
Link: https://patch.msgid.link/20260709122933.4021501-2-zhouchuyi@bytedance.com
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__csd_lock_record() publishes per-CPU diagnostic state (cur_csd,
cur_csd_func, cur_csd_info) that is consumed from a remote CPU by
csd_lock_wait_toolong() via smp_load_acquire(&cur_csd). To order the
matching cur_csd_func/cur_csd_info stores before the cur_csd
publication, the producer issues smp_wmb() before writing cur_csd;
to order the publication before the subsequent callback execution or
CSD unlock, it issues smp_mb() after the write. The clear path
mirrors this with smp_mb() before storing NULL into cur_csd so the
preceding callback/unlock is observed first.
The smp_mb() pair is heavier than what the consumer actually
requires (on x86 each emits a locked full barrier). The consumer
only needs to observe the matching cur_csd_func/cur_csd_info when it
sees a non-NULL cur_csd, and to observe the preceding callback/unlock
when it sees NULL -- both of which a release/acquire pair provides.
The extra two-way ordering enforced by smp_mb() -- that cur_csd
publication be observed before callback execution or unlock becomes
visible -- would only matter if cur_csd were an exact live-state
marker. csd_lock_wait_toolong() does not treat it that way: it
snapshots cur_csd via smp_load_acquire() and then prints / dumps /
re-IPIs without an RCU-style stall-ended recheck, so the diagnostic
already tolerates the remote CPU completing its work between snapshot
and report. cur_csd is best-effort context, not a precise stall
boundary.
Replace the smp_wmb() + plain store + smp_mb() in the publish path,
and the smp_mb() + plain store in the clear path, with
smp_store_release(). This pairs with the smp_load_acquire() in
csd_lock_wait_toolong(): preceding cur_csd_func/cur_csd_info stores
become visible before a remote reader observes the non-NULL
publication, and any preceding callback/unlock becomes visible before
a reader observes the NULL clear.
Signed-off-by: Usama Arif <usama.arif@linux.dev>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Dmitry Ilvokhin <d@ilvokhin.com>
Link: https://patch.msgid.link/20260629105745.1696683-1-usama.arif@linux.dev
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git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace
Pull tracing updates from Steven Rostedt:
- Fix printf format warning for bprintf
sunrpc uses a trace_printk() that triggers a printf warning during
the compile. Move the __printf() attribute around for when debugging
is not enabled the warning will go away
- Remove redundant check for EVENT_FILE_FL_FREED in
event_filter_write()
The FREED flag is checked in the call to event_file_file() and then
checked again right afterward, which is unneeded
- Clean up event_file_file() and event_file_data() helpers
These helper functions played a different role in the past, but now
with eventfs, the READ_ONCE() isn't needed. Simplify the code a bit
and also add a warning to event_file_data() if the file or its data
is not present
- Remove updating file->private_data in tracing open
All access to the file private data is handled by the helper
functions, which do not use file->private_data. Stop updating it on
open
- Show ENUM names in function arguments via BTF in function tracing
When showing the function arguments when func-args option is set for
function tracing, if one of the arguments is found to be an enum,
show the name of the enum instead of its number
- Add new trace_call__##name() API for tracepoints
Tracepoints are enabled via static_branch() blocks, where when not
enabled, there's only a nop that is in the code where the execution
will just skip over it. When tracing is enabled, the nop is converted
to a direct jump to the tracepoint code. Sometimes more calculations
are required to be performed to update the parameters of the
tracepoint. In this case, trace_##name##_enabled() is called which is
a static_branch() that gets enabled only when the tracepoint is
enabled. This allows the extra calculations to also be skipped by the
nop:
if (trace_foo_enabled()) {
x = bar();
trace_foo(x);
}
Where the x=bar() is only performed when foo is enabled. The problem
with this approach is that there's now two static_branch() calls. One
for checking if the tracepoint is enabled, and then again to know if
the tracepoint should be called. The second one is redundant
Introduce trace_call__foo() that will call the foo() tracepoint
directly without doing a static_branch():
if (trace_foo_enabled()) {
x = bar();
trace_call__foo();
}
- Update various locations to use the new trace_call__##name() API
- Move snapshot code out of trace.c
Cleaning up trace.c to not be a "dump all", move the snapshot code
out of it and into a new trace_snapshot.c file
- Clean up some "%*.s" to "%*s"
- Allow boot kernel command line options to be called multiple times
Have options like:
ftrace_filter=foo ftrace_filter=bar ftrace_filter=zoo
Equal to:
ftrace_filter=foo,bar,zoo
- Fix ipi_raise event CPU field to be a CPU field
The ipi_raise target_cpus field is defined as a __bitmask(). There is
now a __cpumask() field definition. Update the field to use that
- Have hist_field_name() use a snprintf() and not a series of strcat()
It's safer to use snprintf() that a series of strcat()
- Fix tracepoint regfunc balancing
A tracepoint can define a "reg" and "unreg" function that gets called
before the tracepoint is enabled, and after it is disabled
respectively. But on error, after the "reg" func is called and the
tracepoint is not enabled, the "unreg" function is not called to tear
down what the "reg" function performed
- Fix output that shows what histograms are enabled
Event variables are displayed incorrectly in the histogram output
Instead of "sched.sched_wakeup.$var", it is showing
"$sched.sched_wakeup.var" where the '$' is in the incorrect location
- Some other simple cleanups
* tag 'trace-v7.1' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace: (24 commits)
selftests/ftrace: Add test case for fully-qualified variable references
tracing: Fix fully-qualified variable reference printing in histograms
tracepoint: balance regfunc() on func_add() failure in tracepoint_add_func()
tracing: Rebuild full_name on each hist_field_name() call
tracing: Report ipi_raise target CPUs as cpumask
tracing: Remove duplicate latency_fsnotify() stub
tracing: Preserve repeated trace_trigger boot parameters
tracing: Append repeated boot-time tracing parameters
tracing: Remove spurious default precision from show_event_trigger/filter formats
cpufreq: Use trace_call__##name() at guarded tracepoint call sites
tracing: Remove tracing_alloc_snapshot() when snapshot isn't defined
tracing: Move snapshot code out of trace.c and into trace_snapshot.c
mm: damon: Use trace_call__##name() at guarded tracepoint call sites
btrfs: Use trace_call__##name() at guarded tracepoint call sites
spi: Use trace_call__##name() at guarded tracepoint call sites
i2c: Use trace_call__##name() at guarded tracepoint call sites
kernel: Use trace_call__##name() at guarded tracepoint call sites
tracepoint: Add trace_call__##name() API
tracing: trace_mmap.h: fix a kernel-doc warning
tracing: Pretty-print enum parameters in function arguments
...
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When a caller enqueues a work item using schedule_delayed_work() the used
wq is "system_wq" (per-cpu wq) while queue_delayed_work() uses
WORK_CPU_UNBOUND (used when no target CPU is specified). The same applies
to schedule_work() that is using system_wq and queue_work(), which again
makes use of WORK_CPU_UNBOUND.
This lack of consistency cannot be addressed without refactoring the API.
Continue the effort to refactor workqueue APIs, which began with the
introduction of new workqueues and a new alloc_workqueue() flag in:
commit 128ea9f6ccfb ("workqueue: Add system_percpu_wq and system_dfl_wq")
commit 930c2ea566af ("workqueue: Add new WQ_PERCPU flag")
and switch smp_call_on_cpu() to use system_percpu_wq because system_wq is
going away once the ongoing workqueue restructuring is done.
Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://patch.msgid.link/20251110170332.319314-1-marco.crivellari@suse.com
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Replace trace_foo() with the new trace_call__foo() at sites already
guarded by trace_foo_enabled(), avoiding a redundant
static_branch_unlikely() re-evaluation inside the tracepoint.
trace_call__foo() calls the tracepoint callbacks directly without
utilizing the static branch again.
Cc: David Vernet <void@manifault.com>
Cc: Andrea Righi <arighi@nvidia.com>
Cc: Changwoo Min <changwoo@igalia.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Ben Segall <bsegall@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Thomas Gleixner <tglx@kernel.org>
Cc: "Yury Norov [NVIDIA]" <yury.norov@gmail.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Roman Kisel <romank@linux.microsoft.com>
Cc: Joel Fernandes <joelagnelf@nvidia.com>
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
Cc: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://patch.msgid.link/20260323160052.17528-3-vineeth@bitbyteword.org
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Vineeth Pillai (Google) <vineeth@bitbyteword.org>
Assisted-by: Claude:claude-sonnet-4-6
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Thomas Gleixner <tglx@kernel.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
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Both smp_call_function() and smp_call_function_single() use per-CPU
call_single_data_t variable to hold the infamous CSD lock. However,
while smp_call_function() acquires the destination CPU's CSD lock,
smp_call_function_single() instead uses the source CPU's CSD lock.
(These are two separate sets of CSD locks, cfd_data and csd_data,
respectively.)
This otherwise inexplicable pair of choices is explained by their
respective queueing properties. If smp_call_function() where to
use the sending CPU's CSD lock, that would serialize the destination
CPUs' IPI handlers and result in long smp_call_function() latencies,
especially on systems with large numbers of CPUs. For its part, if
smp_call_function_single() were to use the (single) destination CPU's
CSD lock, this would similarly serialize in the case where many CPUs
are sending IPIs to a single "victim" CPU. Plus it would result in
higher levels of memory contention.
Except that if there is no NMI-based stack tracing on a weakly ordered
system where remote unsynchronized stack traces are especially unreliable,
the improved debugging beats the improved queueing. This improved queueing
only matters if a bunch of CPUs are calling smp_call_function_single()
concurrently for a single "victim" CPU, which is not the common case.
Therefore, make smp_call_function_single() use the destination CPU's
csd_data instance in kernels built with CONFIG_CSD_LOCK_WAIT_DEBUG=y
where csdlock_debug_enabled is also set. Otherwise, continue to use
the source CPU's csd_data.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://patch.msgid.link/25c2eb97-77c8-49a5-80ac-efe78dea272c@paulmck-laptop
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smp_call_function_single() and smp_call_function_many_cond() disable
preemption and cache the CPU number via get_cpu().
Use this cached value throughout the function instead of invoking
smp_processor_id() again.
[ tglx: Make the copy&pasta'ed change log match the patch ]
Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
Link: https://patch.msgid.link/20260323193630.640311-4-sshegde@linux.ibm.com
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Add missing kernel-doc comments and rearrange the order of others to
prevent all kernel-doc warnings.
- add function Returns: sections or format existing comments as kernel-doc
- add missing function parameter comments
- use "/**" for smp_call_function_any() and on_each_cpu_cond_mask()
- correct the commented function name for on_each_cpu_cond_mask()
- use correct format for function short descriptions
- add all kernel-doc comments for smp_call_on_cpu()
- remove kernel-doc comments for raw_smp_processor_id() since there is
no prototype for it here (other than !SMP)
- in smp.h, rearrange some lines so that the kernel-doc comments for
smp_processor_id() are immediately before the macro (to prevent
kernel-doc warnings)
- remove "Returns" from smp_call_function() since it doesn't
return a value
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20260310061726.1153764-1-rdunlap@infradead.org
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When governors used during cpuidle try to find the most optimal idle state
for a CPU or a group of CPUs, they are known to quite often fail. One
reason for this is, that they are not taking into account whether there has
been an IPI scheduled for any of the CPUs that are affected by the selected
idle state.
To enable pending IPIs to be taken into account for cpuidle decisions,
introduce a new helper function, cpus_peek_for_pending_ipi().
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
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The smp_call_function_many() kerneldoc comment got out of sync with the
function definition (bool parameter "wait" is incorrectly described as a
bitmask in it), so fix it up by copying the "wait" description from the
smp_call_function() kerneldoc and add information regarding the handling
of the local CPU to it.
Fixes: 49b3bd213a9f ("smp: Fix all kernel-doc warnings")
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
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"boolean" is spelt as "blooean". Fix that.
Signed-off-by: Roman Kisel <romank@linux.microsoft.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250722161818.6139-1-romank@linux.microsoft.com
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git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu
Pull stop-machine documentation updates from Paul McKenney:
- Improve kernel-doc function-header comments
- Document preemption and stop_machine() mutual exclusion (Joel
Fernandes)
* tag 'stop-machine.2025.07.23a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu:
smp: Document preemption and stop_machine() mutual exclusion
stop_machine: Improve kernel-doc function-header comments
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Recently while revising RCU's cpu online checks, there was some discussion
around how IPIs synchronize with hotplug.
Add comments explaining how preemption disable creates mutual exclusion with
CPU hotplug's stop_machine mechanism. The key insight is that stop_machine()
atomically updates CPU masks and flushes IPIs with interrupts disabled, and
cannot proceed while any CPU (including the IPI sender) has preemption
disabled.
[ Apply peterz feedback. ]
Cc: Andrea Righi <arighi@nvidia.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: rcu@vger.kernel.org
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Co-developed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
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Whenever work is enqueued for a remote CPU, smp_call_function_many_cond()
may need to wait for that work to be completed. However, if no work is
enqueued for a remote CPU, because the condition func() evaluated to false
for all CPUs, there is no need to wait.
Set run_remote only if work was enqueued on remote CPUs.
Document the difference between "work enqueued", and "CPU needs to be
woken up"
Suggested-by: Jann Horn <jannh@google.com>
Signed-off-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>
Link: https://lore.kernel.org/all/20250703203019.11331ac3@fangorn
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Defer check for local execution to the actual place where it is needed,
which removes the extra local variable.
Signed-off-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250623000010.10124-5-yury.norov@gmail.com
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smp_call_function_many_cond() opencodes cpumask_any_but().
Signed-off-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250623000010.10124-3-yury.norov@gmail.com
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smp_call_function_any() tries to make a local call as it's the cheapest
option, or switches to a CPU in the same node. If it's not possible, the
algorithm gives up and searches for any CPU, in a numerical order.
Instead, it can search for the best CPU based on NUMA locality, including
the 2nd nearest hop (a set of equidistant nodes), and higher.
sched_numa_find_nth_cpu() does exactly that, and also helps to drop most
of the housekeeping code.
Signed-off-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250623000010.10124-2-yury.norov@gmail.com
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git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu
Pull CSD-lock update from Paul McKenney:
"Allow runtime modification of the csd_lock_timeout and
panic_on_ipistall module parameters"
* tag 'csd-lock.2025.01.28a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu:
locking/csd-lock: make CSD lock debug tunables writable in /sys
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Currently the CSD lock tunables can only be set at boot time in the
kernel commandline, but the way these variables are used means there
is really no reason not to tune them at runtime through /sys.
Make the CSD lock debug tunables tunable through /sys.
Signed-off-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
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In smp_call_function_many_cond(), the local cond_func() is evaluated
after triggering the remote CPU IPIs.
If cond_func() depends on loading shared state updated by other CPU's
IPI handlers func(), then triggering execution of remote CPUs IPI before
evaluating cond_func() may have unexpected consequences.
One example scenario is evaluating a jiffies delay in cond_func(), which
is updated by func() in the IPI handlers. This situation can prevent
execution of periodic cleanup code on the local CPU.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Rik van Riel <riel@surriel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20241203163558.3455535-1-mathieu.desnoyers@efficios.com
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Currently, the CONFIG_CSD_LOCK_WAIT_DEBUG code uses sched_clock() to check
for excessive CSD-lock wait times. This works, but does not guarantee
monotonic timestamps on x86 due to the sched_clock() function's use of
the rdtsc instruction, which does not guarantee ordering. This means
that, given successive calls to sched_clock(), the second might return
an earlier time than the second, that is, time might seem to go backwards.
This can (and does!) result in false-positive CSD-lock wait complaints
claiming almost 2^64 nanoseconds of delay.
Therefore, switch from sched_clock() to ktime_get_mono_fast_ns(), which
does guarantee monotonic timestamps via the rdtsc_ordered() function,
which as the name implies, does guarantee ordered timestamps, at least
in the absence of calls from NMI handlers, which are not involved in
this code path.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Rik van Riel <riel@surriel.com>
Cc: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
Cc: Leonardo Bras <leobras@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
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About 40% of all csd_lock warnings observed in our fleet appear to
be due to sched_clock() going backward in time (usually only a little
bit), resulting in ts0 being larger than ts2.
When the local CPU is at fault, we should print out a message reflecting
that, rather than trying to get the remote CPU's stack trace.
Signed-off-by: Rik van Riel <riel@surriel.com>
Tested-by: "Paul E. McKenney" <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
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Currently, the CSD-lock diagnostics in CONFIG_CSD_LOCK_WAIT_DEBUG=y
kernels are emitted at five-second intervals. Although this has proven
to be a good time interval for the first diagnostic, if the target CPU
keeps interrupts disabled for way longer than five seconds, the ratio
of useful new information to pointless repetition increases considerably.
Therefore, back off the time period for repeated reports of the same
incident, increasing linearly with the number of reports and logarithmicly
with the number of online CPUs.
[ paulmck: Apply Dan Carpenter feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Imran Khan <imran.f.khan@oracle.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Leonardo Bras <leobras@redhat.com>
Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Cc: Rik van Riel <riel@surriel.com>
Reviewed-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
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If a CSD-lock stall goes on long enough, it will cause an RCU CPU
stall warning. This additional warning provides much additional
console-log traffic and little additional information. Therefore,
provide a new csd_lock_is_stuck() function that returns true if there
is an ongoing CSD-lock stall. This function will be used by the RCU
CPU stall warnings to provide a one-line indication of the stall when
this function returns true.
[ neeraj.upadhyay: Apply Rik van Riel feedback. ]
[ neeraj.upadhyay: Apply kernel test robot feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Imran Khan <imran.f.khan@oracle.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Leonardo Bras <leobras@redhat.com>
Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
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The CSD-lock-hold diagnostics from CONFIG_CSD_LOCK_WAIT_DEBUG are
printed in nanoseconds as unsigned long longs, which is a bit obtuse for
human readers when timing bugs result in negative CSD-lock hold times.
Yes, there are some people to whom it is immediately obvious that
18446744073709551615 is really -1, but for the rest of us...
Therefore, print these numbers as signed long longs, making the negative
hold times immediately apparent.
Reported-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Imran Khan <imran.f.khan@oracle.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Leonardo Bras <leobras@redhat.com>
Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Cc: Rik van Riel <riel@surriel.com>
Reviewed-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>
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For CONFIG_DEBUG_OBJECTS_WORK=y kernels sscs.work defined by
INIT_WORK_ONSTACK() is initialized by debug_object_init_on_stack() for
the debug check in __init_work() to work correctly.
But this lacks the counterpart to remove the tracked object from debug
objects again, which will cause a debug object warning once the stack is
freed.
Add the missing destroy_work_on_stack() invocation to cure that.
[ tglx: Massaged changelog ]
Signed-off-by: Zqiang <qiang.zhang1211@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Link: https://lore.kernel.org/r/20240704065213.13559-1-qiang.zhang1211@gmail.com
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Fixes the following two Coccinelle/coccicheck warnings reported by
string_choices.cocci:
opportunity for str_plural(num_cpus)
opportunity for str_plural(num_nodes)
Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Link: https://lore.kernel.org/r/20240508154225.309703-2-thorsten.blum@toblux.com
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git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu
Pull CSD lock update from Paul McKenney:
"This adds a kernel boot parameter that causes the kernel to panic if
one of the call_smp_function() APIs is stalled for more than the
specified duration.
This is useful in deployments in which a clean panic is preferable to
an indefinite stall"
* tag 'csd-lock.2023.10.23a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu:
smp,csd: Throw an error if a CSD lock is stuck for too long
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Pull in upstream to get the fixes so depending changes can be applied.
|
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The CSD lock seems to get stuck in 2 "modes". When it gets stuck
temporarily, it usually gets released in a few seconds, and sometimes
up to one or two minutes.
If the CSD lock stays stuck for more than several minutes, it never
seems to get unstuck, and gradually more and more things in the system
end up also getting stuck.
In the latter case, we should just give up, so the system can dump out
a little more information about what went wrong, and, with panic_on_oops
and a kdump kernel loaded, dump a whole bunch more information about what
might have gone wrong. In addition, there is an smp.panic_on_ipistall
kernel boot parameter that by default retains the old behavior, but when
set enables the panic after the CSD lock has been stuck for more than
the specified number of milliseconds, as in 300,000 for five minutes.
[ paulmck: Apply Imran Khan feedback. ]
[ paulmck: Apply Leonardo Bras feedback. ]
Link: https://lore.kernel.org/lkml/bc7cc8b0-f587-4451-8bcd-0daae627bcc7@paulmck-laptop/
Signed-off-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Imran Khan <imran.f.khan@oracle.com>
Reviewed-by: Leonardo Bras <leobras@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Randy Dunlap <rdunlap@infradead.org>
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call_single_data_t is a size-aligned typedef of struct __call_single_data.
This alignment is desirable in order to have smp_call_function*() avoid
bouncing an extra cacheline in case of an unaligned csd, given this
would hurt performance.
Since the removal of struct request->csd in commit 660e802c76c8
("blk-mq: use percpu csd to remote complete instead of per-rq csd") there
are no current users of smp_call_function*() with unaligned csd.
Change every 'struct __call_single_data' function parameter to
'call_single_data_t', so we have warnings if any new code tries to
introduce an smp_call_function*() call with unaligned csd.
Signed-off-by: Leonardo Bras <leobras@redhat.com>
Reviewed-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20230831063129.335425-1-leobras@redhat.com
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On systems with hundreds of CPUs, if most of the CPUs detect a CSD hang,
then all of these waiting CPUs send an NMI to the destination CPU in
order to dump its backtrace.
Given enough NMIs, the destination CPU will spent much of its time
producing backtraces, thus further delaying that CPU's response to the
original CSD IPI. In the worst case, by the time destination CPU is
done producing all of these backtrace NMIs, the CSD wait timeout will
have elapsed so that the waiters resend their backtrace NMIs again,
further delaying forward progress.
Therefore, to avoid these delays, issue the backtrace NMI only from
the first waiter. The destination CPU's other waiters can make use of
backtrace obtained from the first waiter's NMI.
Signed-off-by: Imran Khan <imran.f.khan@oracle.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juergen Gross <jgross@suse.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
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If a waiter is waiting for CSD lock, its call stack will not change
between first and subsequent hang detection for the same CSD lock.
Therefore, do dump_stack only for first-time detection for a given waiter.
This avoids excessive logging on systems with hundreds of CPUs where
repetitive dump_stack from hundreds of CPUs would otherwise flood the
console.
Signed-off-by: Imran Khan <imran.f.khan@oracle.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juergen Gross <jgross@suse.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
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Add a tracepoint for when a CSD is queued to a remote CPU's
call_single_queue. This allows finding exactly which CPU queued a given CSD
when looking at a csd_function_{entry,exit} event, and also enables us to
accurately measure IPI delivery time with e.g. a synthetic event:
$ echo 'hist:keys=cpu,csd.hex:ts=common_timestamp.usecs' >\
/sys/kernel/tracing/events/smp/csd_queue_cpu/trigger
$ echo 'csd_latency unsigned int dst_cpu; unsigned long csd; u64 time' >\
/sys/kernel/tracing/synthetic_events
$ echo \
'hist:keys=common_cpu,csd.hex:'\
'time=common_timestamp.usecs-$ts:'\
'onmatch(smp.csd_queue_cpu).trace(csd_latency,common_cpu,csd,$time)' >\
/sys/kernel/tracing/events/smp/csd_function_entry/trigger
$ trace-cmd record -e 'synthetic:csd_latency' hackbench
$ trace-cmd report
<...>-467 [001] 21.824263: csd_queue_cpu: cpu=0 callsite=try_to_wake_up+0x2ea func=sched_ttwu_pending csd=0xffff8880076148b8
<...>-467 [001] 21.824280: ipi_send_cpu: cpu=0 callsite=try_to_wake_up+0x2ea callback=generic_smp_call_function_single_interrupt+0x0
<...>-489 [000] 21.824299: csd_function_entry: func=sched_ttwu_pending csd=0xffff8880076148b8
<...>-489 [000] 21.824320: csd_latency: dst_cpu=0, csd=18446612682193848504, time=36
Suggested-by: Valentin Schneider <vschneid@redhat.com>
Signed-off-by: Leonardo Bras <leobras@redhat.com>
Tested-and-reviewed-by: Valentin Schneider <vschneid@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230615065944.188876-7-leobras@redhat.com
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The recently added ipi_send_{cpu,cpumask} tracepoints allow finding sources
of IPIs targeting CPUs running latency-sensitive applications.
For NOHZ_FULL CPUs, all IPIs are interference, and those tracepoints are
sufficient to find them and work on getting rid of them. In some setups
however, not *all* IPIs are to be suppressed, but long-running IPI
callbacks can still be problematic.
Add a pair of tracepoints to mark the start and end of processing a CSD IPI
callback, similar to what exists for softirq, workqueue or timer callbacks.
Signed-off-by: Leonardo Bras <leobras@redhat.com>
Tested-and-reviewed-by: Valentin Schneider <vschneid@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230615065944.188876-5-leobras@redhat.com
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No point in keeping them around.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Michael Kelley <mikelley@microsoft.com>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Tested-by: Helge Deller <deller@gmx.de> # parisc
Tested-by: Guilherme G. Piccoli <gpiccoli@igalia.com> # Steam Deck
Link: https://lore.kernel.org/r/20230512205255.551974164@linutronix.de
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(Ab)use the trace_ipi_send_cpu*() family to trace all
smp_function_call*() invocations, not only those that result in an
actual IPI.
The queued entries log their callback function while the actual IPIs
are traced on generic_smp_call_function_single_interrupt().
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
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Because copying cpumasks around when targeting a single CPU is a bit
daft...
Tested-and-reviewed-by: Valentin Schneider <vschneid@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20230322103004.GA571242%40hirez.programming.kicks-ass.net
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Context
=======
The newly-introduced ipi_send_cpumask tracepoint has a "callback" parameter
which so far has only been fed with NULL.
While CSD_TYPE_SYNC/ASYNC and CSD_TYPE_IRQ_WORK share a similar backing
struct layout (meaning their callback func can be accessed without caring
about the actual CSD type), CSD_TYPE_TTWU doesn't even have a function
attached to its struct. This means we need to check the type of a CSD
before eventually dereferencing its associated callback.
This isn't as trivial as it sounds: the CSD type is stored in
__call_single_node.u_flags, which get cleared right before the callback is
executed via csd_unlock(). This implies checking the CSD type before it is
enqueued on the call_single_queue, as the target CPU's queue can be flushed
before we get to sending an IPI.
Furthermore, send_call_function_single_ipi() only has a CPU parameter, and
would need to have an additional argument to trickle down the invoked
function. This is somewhat silly, as the extra argument will always be
pushed down to the function even when nothing is being traced, which is
unnecessary overhead.
Changes
=======
send_call_function_single_ipi() is only used by smp.c, and is defined in
sched/core.c as it contains scheduler-specific ops (set_nr_if_polling() of
a CPU's idle task).
Split it into two parts: the scheduler bits remain in sched/core.c, and the
actual IPI emission is moved into smp.c. This lets us define an
__always_inline helper function that can take the related callback as
parameter without creating useless register pressure in the non-traced path
which only gains a (disabled) static branch.
Do the same thing for the multi IPI case.
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230307143558.294354-8-vschneid@redhat.com
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Accessing the call_single_queue hasn't involved a spinlock since 2014:
6897fc22ea01 ("kernel: use lockless list for smp_call_function_single")
The llist operations (namely cmpxchg() and xchg()) provide similar ordering
guarantees, update the comment to lessen confusion.
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230307143558.294354-7-vschneid@redhat.com
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