linux-kernel - Re: [patch V2 0/3] signals: Allow caching one sigqueue object per task

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Message-ID: <m1blbpjswd.fsf@fess.ebiederm.org>
Date:   Thu, 11 Mar 2021 15:13:22 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        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>,
        Matt Fleming <matt@...eblueprint.co.uk>
Subject: Re: [patch V2 0/3] signals: Allow caching one sigqueue object per task

Thomas Gleixner <tglx@...utronix.de> writes:

> This is a follow up to the initial submission which can be found here:
>
>   https://lore.kernel.org/r/20210303142025.wbbt2nnr6dtgwjfi@linutronix.de
>
> Signal sending requires a kmem cache allocation at the sender side and the
> receiver hands it back to the kmem cache when consuming the signal.
>
> This works pretty well even for realtime workloads except for the case when
> the kmem cache allocation has to go into the slow path which is rare but
> happens.
>
> Preempt-RT carries a patch which allows caching of one sigqueue object per
> task. The object is not preallocated. It's cached when the task receives a
> signal. The cache is freed when the task exits.

I am probably skimming fast and missed your explanation but is there
a reason the caching is per task (aka thread) and not per signal_struct
(aka process)?

My sense is most signal delivery is per process.  Are realtime workloads
that extensively use pthread_sigqueue?  The ordinary sigqueue interface
only allows targeting a process.

Mostly I am just trying to get a sense of the workloads that are
improved by this.

Eric