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

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Message-ID: <20140804075528.GI9918@twins.programming.kicks-ass.net>
Date:	Mon, 4 Aug 2014 09:55:28 +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 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? 

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