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

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Message-ID: <877dmc2l9z.fsf@nanos.tec.linutronix.de>
Date:   Fri, 12 Mar 2021 21:02:16 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
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

On Thu, Mar 11 2021 at 15:13, Eric W. Biederman wrote:
> 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.

Unfortunately they use both. The majority is probably process based.

Thanks,

        tglx