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

From: ebiederm@xmission.com (Eric W. Biederman)
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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
Date: Thu, 04 Mar 2021 13:01:04 -0600	[thread overview]
Message-ID: <m1czwe7ngv.fsf@fess.ebiederm.org> (raw)
In-Reply-To: <20210304081142.digtkddajkadwwq5@linutronix.de> (Sebastian Andrzej Siewior's message of "Thu, 4 Mar 2021 09:11:42 +0100")

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

> On 2021-03-03 16:09:05 [-0600], Eric W. Biederman wrote:
>> Sebastian Andrzej Siewior <bigeasy@linutronix.de> writes:
>> 
>> > 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 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