Messages in this thread | | | Date | Tue, 14 Jan 2014 17:00:56 -0800 | From | Andrew Morton <> | Subject | Re: [RFC 3/3] mutex: When there is no owner, stop spinning after too many tries |
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On Tue, 14 Jan 2014 16:33:10 -0800 Jason Low <jason.low2@hp.com> wrote:
> When running workloads that have high contention in mutexes on an 8 socket > machine, spinners would often spin for a long time with no lock owner. > > One of the potential reasons for this is because a thread can be preempted > after clearing lock->owner but before releasing the lock, or preempted after > acquiring the mutex but before setting lock->owner. In those cases, the > spinner cannot check if owner is not on_cpu because lock->owner is NULL.
That sounds like a very small window. And your theory is that this window is being hit sufficiently often to impact aggregate runtime measurements, which sounds improbable to me?
> A solution that would address the preemption part of this problem would > be to disable preemption between acquiring/releasing the mutex and > setting/clearing the lock->owner. However, that will require adding overhead > to the mutex fastpath.
preempt_disable() is cheap, and sometimes free.
Have you confirmed that the preempt_disable() approach actually fixes the performance issues? If it does then this would confirm your "potential reason" hypothesis. If it doesn't then we should be hunting further for the explanation.
> The solution used in this patch is to limit the # of times thread can spin on > lock->count when !owner. > > The threshold used in this patch for each spinner was 128, which appeared to > be a generous value, but any suggestions on another method to determine > the threshold are welcomed.
It's a bit hacky, isn't it? If your "owner got preempted in the window" theory is correct then I guess this is reasonableish. But if !owner is occurring for other reasons then perhaps there are better solutions.
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