linux-kernel - Re: [PATCH 1/7] locking/rwsem: don't resched at the end of optimistic spinning

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Date:	Mon, 4 Aug 2014 22:48:24 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@...com>
Cc:	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	linux-api@...r.kernel.org, linux-doc@...r.kernel.org,
	Davidlohr Bueso <davidlohr@...com>,
	Jason Low <jason.low2@...com>,
	Scott J Norton <scott.norton@...com>
Subject: Re: [PATCH 1/7] locking/rwsem: don't resched at the end of
 optimistic spinning

On Mon, Aug 04, 2014 at 02:36:35PM -0400, Waiman Long wrote:
> On 08/04/2014 03:55 AM, Peter Zijlstra wrote:
> >On Sun, Aug 03, 2014 at 10:36:16PM -0400, Waiman Long wrote:
> >>For a fully preemptive kernel, a call to preempt_enable() could
> >>potentially trigger a task rescheduling event. In the case of rwsem
> >>optimistic spinning, the task has either gotten the lock or is going
> >>to sleep soon. So there is no point to do rescheduling here.
> >Uh what? Why shouldn't we preempt if we've gotten the lock? What if a
> >FIFO task just woke up?
> 
> I didn't mean that we shouldn't preempt if there is a higher priority task.
> I am sure that there will be other preemption points along the way that a
> higher priority task can take over the CPU. I just want to say that doing it
> here may not be the best place especially if the task is going to sleep
> soon.
> 
> If you think this patch does not make sense, I can remove it as other
> patches in the set has no dependency on this one.

Yeah, its actively harmful, you delay preemption by an unspecified
amount of time in case of the spin-acquire. We've had such bugs in -rt
and they're not fun.

Basically the only time you should use no_resched is if the very next
statement is schedule().
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