linux-kernel - Re: [PATCH v2] lib/smp_processor_id: Don't use cpumask

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Date:   Fri, 4 Oct 2019 12:32:55 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Juri Lelli <jlelli@...hat.com>
Subject: Re: [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@...hat.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@...hat.com>

Thanks,

Juri