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SubjectRe: [PATCH v2] lib/smp_processor_id: Don't use cpumask_equal()
Hi,

On 03/10/19 16:36, Waiman Long wrote:
> 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 <longman@redhat.com>
> ---
> 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 60ba93fc42ce..bd9571653288 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;

Makes sense to me.

Reviewed-by: Juri Lelli <juri.lelli@redhat.com>

Thanks,

Juri

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