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

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Message-ID: <87tupr55ea.fsf@nanos.tec.linutronix.de>
Date:   Thu, 04 Mar 2021 16:02:05 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     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 04 2021 at 09:11, Sebastian Andrzej Siewior wrote:
> On 2021-03-03 16:09:05 [-0600], Eric W. Biederman wrote:
>> Sebastian Andrzej Siewior <bigeasy@...utronix.de> writes:
>> 
>> > From: Thomas Gleixner <tglx@...utronix.de>
>> >
>> > Allow realtime tasks to cache one sigqueue in task struct. This avoids an
>> > allocation which can increase the latency or fail.
>> > Ideally the sigqueue is cached after first successful delivery and will be
>> > available for next signal delivery. This works under the assumption that the RT
>> > task has never an unprocessed signal while a one is about to be queued.
>> >
>> > The caching is not used for SIGQUEUE_PREALLOC because this kind of sigqueue is
>> > handled differently (and not used for regular signal delivery).
>> 
>> What part of this is about real time tasks?  This allows any task
>> to cache a sigqueue entry.
>
> It is limited to realtime tasks (SCHED_FIFO/RR/DL):
>
> +static void __sigqueue_cache_or_free(struct sigqueue *q)
> +{
> …
> +	if (!task_is_realtime(current) || !sigqueue_add_cache(current, q))
> +		kmem_cache_free(sigqueue_cachep, q);
> +}

We could of course do the caching unconditionally for all tasks.

Thanks,

        tglx