linux-kernel - Re: [PATCH RT] kernel/sched: Don't recompute cpumask weight in migrate_enable_update_cpus

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Date:   Sat, 12 Oct 2019 02:06:16 -0500
From:   Scott Wood <swood@...hat.com>
To:     Waiman Long <longman@...hat.com>, linux-kernel@...r.kernel.org,
        linux-rt-users@...r.kernel.org
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Juri Lelli <jlelli@...hat.com>
Subject: Re: [PATCH RT] kernel/sched: Don't recompute cpumask weight in
 migrate_enable_update_cpus_allowed()

On Fri, 2019-10-11 at 10:09 -0400, Waiman Long wrote:
> At each invocation of rt_spin_unlock(), cpumask_weight() is called
> via migrate_enable_update_cpus_allowed() to recompute the weight of
> cpus_mask which doesn't change that often.
> 
> The following is a sample output of perf-record running the testpmd
> microbenchmark on an RT kernel:
> 
>   34.77%   1.65%  testpmd  [kernel.kallsyms]  [k] rt_spin_unlock
>   34.32%   2.52%  testpmd  [kernel.kallsyms]  [k] migrate_enable
>   21.76%  21.76%  testpmd  [kernel.kallsyms]  [k] __bitmap_weight
> 
> By adding an extra variable to keep track of the weight of cpus_mask,
> we could eliminate the frequent call to cpumask_weight() and replace
> it with simple assignment.

Can you try this with my migrate disable patchset (which makes
amigrate_enable_update_cpus_allowed() be called much less often) and see if
caching nr_cpus_allowed still makes a difference?

-Scott