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Date:   Thu, 4 Mar 2021 09:11:42 +0100
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Matt Fleming <matt@codeblueprint.co.uk>
Subject: Re: [PATCH] signal: Allow RT tasks to cache one sigqueue struct
Message-ID: <20210304081142.digtkddajkadwwq5@linutronix.de>
References: <20210303142025.wbbt2nnr6dtgwjfi@linutronix.de>
 <m1zgzj7uv2.fsf@fess.ebiederm.org>
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On 2021-03-03 16:09:05 [-0600], Eric W. Biederman wrote:
> Sebastian Andrzej Siewior <bigeasy@linutronix.de> writes:
>=20
> > From: Thomas Gleixner <tglx@linutronix.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 tha=
t 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 sigq=
ueue is
> > handled differently (and not used for regular signal delivery).
>=20
> 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)
+{
=E2=80=A6
+	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