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

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Date:   Fri, 4 Oct 2019 08:30:59 -0400
From:   Waiman Long <longman@...hat.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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>,
        Juri Lelli <jlelli@...hat.com>
Subject: Re: [PATCH v2] lib/smp_processor_id: Don't use cpumask_equal()

On 10/4/19 5:20 AM, Sebastian Andrzej Siewior wrote:
> On 2019-10-03 16:36:08 [-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.
> using a simple integer comparison is still more efficient than what
> __HAVE_ARCH_MEMCMP can offer.
You are right. My main point is to try to avoid using cpumask_equal() in
performance critical path irrespective of this patch.
>> Signed-off-by: Waiman Long <longman@...hat.com>
> Acked-by:  Sebastian Andrzej Siewior <bigeasy@...utronix.de>
>
> Sebastian

Cheers,
Longman