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

[Jason Low <jason.low2@hp.com>]

On Mon, 2014-08-04 at 22:48 +0200, 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().

Right, we actually added an extra resched check at essentially the same
point in the mutex spinning code  :)