linux-kernel - Re: [RFC 3/4] Disable scheduler tick when we are running SCHED

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Message-ID: <alpine.LFD.2.00.1009081515350.2477@localhost6.localdomain6>
Date:	Wed, 8 Sep 2010 15:21:44 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Jan Blunck <jblunck@...e.de>,
	Linux-Kernel Mailinglist <linux-kernel@...r.kernel.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	Sven-Thorsten Dietrich <sdietrich@...ell.com>,
	Michael Galbraith <MGalbraith@...ell.com>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [RFC 3/4] Disable scheduler tick when we are running SCHED_FIFO
 tasks

On Wed, 8 Sep 2010, Peter Zijlstra wrote:

> On Wed, 2010-09-08 at 14:29 +0200, Jan Blunck wrote:
> > This patch is disabling the scheduler tick to go off when there is a task
> > with SCHED_FIFO policy running. Since these tasks are not timesliced anyway
> > we only care about timers, softirqs and such stuff just like when we disable
> > the tick during idle periods.
> > 
> 
> Also, doesn't this break any and all jiffies users?

Only on UP. On SMP we hand off the do_timer duty to some other core.
 
> And you need to restart the tick on call_rcu() and everything else that
> requires the tick for processing.

Not only this. If the task enqueues a timer_list timer via a syscall
or an interrupt/softirq enqueues a timer_list timer while the tick is
off then this timer will not fire until the task goes back into
schedule.

That approach is way too naive. There is a boatload of subtle wreckage
waiting and it needs careful analysis of this to work. Frederic is
looking into this already.

Thanks,

	tglx
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