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

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Message-ID: <20191007123909.GE2294@hirez.programming.kicks-ass.net>
Date:   Mon, 7 Oct 2019 14:39:09 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...nel.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()

On Thu, Oct 03, 2019 at 04:36:08PM -0400, 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>

Thanks!