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

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Date:   Fri, 18 Oct 2019 14:18:43 -0400
From:   Waiman Long <longman@...hat.com>
To:     Scott Wood <swood@...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 10/12/19 3:06 AM, Scott Wood wrote:
> 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

With lazy migrate_disable, I do think my patch will no longer be necessary.

Thanks,
Longman