Messages in this thread | | | Date | Tue, 05 Aug 2014 13:54:37 -0400 | From | Waiman Long <> | Subject | Re: [PATCH 1/7] locking/rwsem: don't resched at the end of optimistic spinning |
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On 08/04/2014 04:48 PM, Peter Zijlstra wrote: > 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().
Thank for the clarification. I will remove patch 1 from the patch set.
-Longman
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