Re: [RFC 3/3] mutex: When there is no owner, stop spinning after too many tries

[Andrew Morton <akpm@linux-foundation.org>]

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.