linux-kernel - Re: [PATCH] signal: Allow RT tasks to cache one sigqueue struct

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Message-ID: <877dmd4vsf.fsf@nanos.tec.linutronix.de>
Date:   Thu, 11 Mar 2021 15:20:00 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Matt Fleming <matt@...eblueprint.co.uk>
Subject: Re: [PATCH] signal: Allow RT tasks to cache one sigqueue struct

On Thu, Mar 11 2021 at 13:45, Thomas Gleixner wrote:

> On Thu, Mar 11 2021 at 00:56, Thomas Gleixner wrote:
>> Rant aside, there is no massive benefit of doing that caching in
>> general, but there is not much of a downside either and for particular
>> use cases it's useful even outside of PREEMPT_RT.
>>
>> IMO, having it there unconditionally is better than yet another special
>> cased hackery.
>
> Just did some micro instrumentation to measure the time spent in
> __sigqueue_alloc/free() with and without the caching.
>
> Unsurprisingly that results in a time reduction of ~67% saving about 3us
> per alloc/free pair. Not hugely relevant for a kernel build but for
> anything which is signal heavy it'll make an difference.

That's all fastpath allocations and nothing which hit the slow path,
which would be way worse.

Thanks,

        tglx