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Subject[tip: locking/core] lib/smp_processor_id: Don't use cpumask_equal()
The following commit has been merged into the locking/core branch of tip:

Commit-ID: e950cca3f3c40902a052a78a36b3fac1f8a62d19
Author: Waiman Long <>
AuthorDate: Thu, 03 Oct 2019 16:36:08 -04:00
Committer: Ingo Molnar <>
CommitterDate: Wed, 09 Oct 2019 12:46:10 +02:00

lib/smp_processor_id: Don't use cpumask_equal()

The check_preemption_disabled() function uses cpumask_equal() to see
if the task is bounded to the current CPU only. cpumask_equal() calls
memcmp() to do the comparison. As x86 doesn't have __HAVE_ARCH_MEMCMP,
the slow memcmp() function in lib/string.c is used.

On a RT kernel that call check_preemption_disabled() very frequently,
below is the perf-record output of a certain microbenchmark:

42.75% 2.45% testpmd [kernel.kallsyms] [k] check_preemption_disabled
40.01% 39.97% testpmd [kernel.kallsyms] [k] memcmp

We should avoid calling memcmp() in performance critical path. So the
cpumask_equal() call is now replaced with an equivalent simpler check.

Signed-off-by: Waiman Long <>
Signed-off-by: Peter Zijlstra (Intel) <>
Reviewed-by: Juri Lelli <>
Acked-by: Sebastian Andrzej Siewior <>
Cc: Linus Torvalds <>
Cc: Masami Hiramatsu <>
Cc: Peter Zijlstra <>
Cc: Thomas Gleixner <>
Signed-off-by: Ingo Molnar <>
lib/smp_processor_id.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/smp_processor_id.c b/lib/smp_processor_id.c
index 60ba93f..bd95716 100644
--- a/lib/smp_processor_id.c
+++ b/lib/smp_processor_id.c
@@ -23,7 +23,7 @@ unsigned int check_preemption_disabled(const char *what1, const char *what2)
* Kernel threads bound to a single CPU can safely use
* smp_processor_id():
- if (cpumask_equal(current->cpus_ptr, cpumask_of(this_cpu)))
+ if (current->nr_cpus_allowed == 1)
goto out;

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