linux-kernel - Re: [PATCH v2] sched/rt: Clean up usage of rt

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Message-ID: <20240529105528.9QBTCqCr@linutronix.de>
Date: Wed, 29 May 2024 12:55:28 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Qais Yousef <qyousef@...alina.io>
Cc: Ingo Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Daniel Bristot de Oliveira <bristot@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	linux-mm@...ck.org, Phil Auld <pauld@...hat.com>
Subject: Re: [PATCH v2] sched/rt: Clean up usage of rt_task()

On 2024-05-29 11:34:09 [+0100], Qais Yousef wrote:
> > behaviour. But then it is insistent which matters only in the RT case.
> > Puh. Any sched folks regarding policy?
> 
> I am not sure I understood you here. Could you rephrase please?

Right now a SCHED_OTHER task boosted to a realtime priority gets
slack=0. In the !RT scenario everything is fine.
For RT the slack=0 also happens but the init of the timer looks at the
policy instead at the possible boosted priority and uses a different
clock attribute. This can lead to a delayed wake up (so avoiding the
slack does not solve the problem).

This is not consistent because IMHO the clock setup & slack should be
handled equally. So I am asking the sched folks for a policy and I am
leaning towards looking at task-policy in this case instead of prio
because you shouldn't do anything that can delay.

Sebastian