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

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

On Tue, 2014-01-14 at 17:00 -0800, Andrew Morton wrote:
> 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.

Using Ingo's test-mutex application (http://lkml.org/lkml/2006/1/8/50)
which can also generate high mutex contention, the preempt_disable()
approach did provide approximately a 4% improvement at 160 threads, but
not nearly the 25+% I was seeing with this patchset. So, it looks like
preemption is not the main cause of the problem then.

Thanks,
Jason