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

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Message-ID: <m1czwe7ngv.fsf@fess.ebiederm.org>
Date:   Thu, 04 Mar 2021 13:01:04 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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

Sebastian Andrzej Siewior <bigeasy@...utronix.de> writes:

> 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);
> +}

I see now.  I was looking for it somewhere in the allocation side.
Oleg's suggestion of simply adding a few additional lines to
__sigqueue_free would have made this stand out more.

A __sigqueue_free that takes the relevant task_struct instead of always
assuming current would be nice here.


>> 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?

No I just missed the task_is_realtime limitation.

Eric