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From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <longman@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel@vger.kernel.org,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Juri Lelli <jlelli@redhat.com>
Subject: Re: [PATCH v2] lib/smp_processor_id: Don't use cpumask_equal()
Date: Mon, 7 Oct 2019 14:39:09 +0200	[thread overview]
Message-ID: <20191007123909.GE2294@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20191003203608.21881-1-longman@redhat.com>

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@redhat.com>

Thanks!

  parent reply	other threads:[~2019-10-07 12:39 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-03 20:36 [PATCH v2] lib/smp_processor_id: Don't use cpumask_equal() Waiman Long
2019-10-04  9:20 ` Sebastian Andrzej Siewior
2019-10-04 12:30   ` Waiman Long
2019-10-04 10:32 ` Juri Lelli
2019-10-07 12:39 ` Peter Zijlstra [this message]
2019-10-09 12:59 ` [tip: locking/core] " tip-bot2 for Waiman Long

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