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

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

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);
+}

> 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