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

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Message-ID: <20210304081142.digtkddajkadwwq5@linutronix.de>
Date:   Thu, 4 Mar 2021 09:11:42 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        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 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);
+}

> Either the patch is buggy or the description is.  Overall caching one
> sigqueue entry doesn't look insane. But it would help to have a clear
> description of what is going on.

Does this clear things up or is my logic somehow broken here?

> Eric

Sebastian