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

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Message-ID: <522FCE9D.4010707@amacapital.net>
Date:	Tue, 10 Sep 2013 18:59:57 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Peter Zijlstra <peterz@...radead.org>
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 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?

--Andy

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