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

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Message-ID: <20191004092048.l2jeutbrffnwfol2@linutronix.de>
Date:   Fri, 4 Oct 2019 11:20:48 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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>,
        Juri Lelli <jlelli@...hat.com>
Subject: Re: [PATCH v2] lib/smp_processor_id: Don't use cpumask_equal()

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.

> Signed-off-by: Waiman Long <longman@...hat.com>
Acked-by:  Sebastian Andrzej Siewior <bigeasy@...utronix.de>

Sebastian