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 :) -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
