From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andy Lutomirski <luto@amacapital.net>
Subject: Re: [PATCH 2/7] sched: Add NEED_RESCHED to the preempt_count
Date: Tue, 10 Sep 2013 18:59:57 -0700
Message-ID: <522FCE9D.4010707@amacapital.net>
References: <20130910130811.507933095@infradead.org> <20130910132011.116593485@infradead.org>
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To: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>, Ingo Molnar <mingo@kernel.org>, Andi Kleen <ak@linux.intel.com>, Peter Anvin <hpa@zytor.com>, Mike Galbraith <bitbucket@online.de>, Thomas Gleixner <tglx@linutronix.de>, Arjan van de Ven <arjan@linux.intel.com>, Frederic Weisbecker <fweisbec@gmail.com>, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org
List-Id: linux-arch.vger.kernel.org

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