linux-kernel - Re: [PATCH 2/7] sched: Add NEED_RESCHED to the preempt

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Date:	Wed, 11 Sep 2013 10:25:30 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andi Kleen <ak@...ux.intel.com>, Peter Anvin <hpa@...or.com>,
	Mike Galbraith <bitbucket@...ine.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH 2/7] sched: Add NEED_RESCHED to the preempt_count

On Tue, Sep 10, 2013 at 06:59:57PM -0700, Andy Lutomirski wrote:
> On 09/10/2013 06:08 AM, Peter Zijlstra wrote:
> > In order to combine the preemption and need_resched test we need to
> > fold the need_resched information into the preempt_count value.
> > 
> > We keep the existing TIF_NEED_RESCHED infrastructure in place but at 3
> > sites test it and fold its value into preempt_count; namely:
> > 
> >  - resched_task() when setting TIF_NEED_RESCHED on the current task
> >  - scheduler_ipi() when resched_task() sets TIF_NEED_RESCHED on a
> >                    remote task it follows it up with a reschedule IPI
> >                    and we can modify the cpu local preempt_count from
> >                    there.
> >  - cpu_idle_loop() for when resched_task() found tsk_is_polling().
> 
> 
> It looks like the intel_idle code can get confused if TIF_NEED_RESCHED
> is set but the preempt resched bit is not -- the need_resched call
> between monitor and mwait won't notice TIF_NEED_RESCHED.
> 
> Is this condition possible?

Ah indeed, I'll have to go find all idle loops out there it seems. Now
if only they were easy to spot :/

I was hoping the generic idle thing was good enough, apparently not
quite. Thanks for spotting that.
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