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

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Date:   Thu, 04 Mar 2021 13:04:08 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        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

Thomas Gleixner <tglx@...utronix.de> writes:

> 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.

Is there any advantage to only doing this for realtime tasks?

If not it probably makes sense to do the caching for all tasks.

I am wondering if we want to count the cached sigqueue structure to the
users rt signal rlimit?

Eric