Re: [RFC][PATCH v2 5/5] mutex: Give spinners a chance tospin_on_owner if need_resched() triggered while queued

[Jason Low]

On Tue, 2014-01-28 at 22:07 +0100, Peter Zijlstra wrote:
> On Tue, Jan 28, 2014 at 11:13:16AM -0800, Jason Low wrote:
> > Before a thread attempts mutex spin on owner, it is first added to a queue
> > using an MCS lock so that only 1 thread spins on owner at a time. However, when
> > the spinner is queued, it is unable to check if it needs to reschedule and
> > will remain on the queue. This could cause the spinner to spin longer
> > than its allocated time. 
> 
> what allocated time?

Hi Peter,

By "spin longer than its allocated time", I meant to say that the thread
can continue spinning/waiting in the MCS queue after need_resched() has
been set.

> > However, once it is the spinner's turn to spin on
> > owner, it would immediately go to slowpath if it need_resched() and gets no spin
> > time. In these situations, not only does the spinner take up more time for a
> > chance to spin for the mutex, it also ends up not getting to spin once it
> > gets its turn.
> > 
> > One solution would be to exit the MCS queue and go to mutex slowpath if
> > need_resched(). However, that may require a lot of overhead. For instance, if a
> > thread at the end of the queue need_resched(), in order to remove it from the
> > queue, we would have to unqueue and requeue all other spinners in front of it.
> 
> If you can do that, you can also walk the list and find prev and cmpxchg
> the entry out. But I don't think you can do either, as we simply don't
> have a head pointer.
> 
> > This RFC patch tries to address the issue in another context by avoiding
> > situations where spinners immediately get sent to the slowpath on
> > need_resched() upon getting to spin. 
> 
> > We will first check for need_resched()
> > right after acquiring the MCS lock. If need_resched() is true, then
> > need_resched() triggered while the thread is waiting in the MCS queue (since
> > patch 1 makes the spinner check for need_resched() before entering the queue).
> 
> > In this case, we will allow the thread to have at least 1 try to do
> > mutex_spin_on_owner() regardless of need_resched(). 
> 
> No! We should absolutely not ever willfully ignore need_resched(). Esp.
> not for unbounded spins.

Ok. This was sort of a proof of concept patch to illustrate the type of
performance gains we can get by addressing this issue.

> > This patch also removes
> > the need_resched() in the outer loop in case we require a few extra spins to
> > observe lock->count == 1, and patch 4 ensures we won't be spinning with
> > lock owner preempted.
> > 
> > And if the need_resched() check after acquiring the MCS lock is false, then
> > we won't give the spinner any extra spinning.
> 
> But urgh, nasty problem. Lemme ponder this a bit.

Thanks,
Jason


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