Re: [PATCH v2] lib/smp_processor_id: Don't use cpumask_equal()

[Juri Lelli <juri.lelli@redhat.com>]

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