Re: [PATCH 00/11] locking/ww_mutex: Keep sorted wait list to avoid stampedes

From: Chris Wilson
Date: Wed Nov 30 2016 - 07:20:45 EST

On Wed, Nov 30, 2016 at 12:52:28PM +0100, Nicolai Hähnle wrote:
> On 30.11.2016 10:40, Chris Wilson wrote:
> >On Mon, Nov 28, 2016 at 01:20:01PM +0100, Nicolai Hähnle wrote:
> >>I've included timings taken from a contention-heavy stress test to some of
> >>the patches. The stress test performs actual GPU operations which take a
> >>good chunk of the wall time, but even so, the series still manages to
> >>improve the wall time quite a bit.
> >
> >In looking at your contention scenarios, what was the average/max list
> >size? Just wondering if it makes sense to use an rbtree + first_waiter
> >instead of a sorted list from the start.
> I haven't measured this with the new series; previously, while I was
> debugging the deadlock on older kernels, I occasionally saw wait
> lists of up to ~20 tasks, spit-balling the average over all the
> deadlock cases I'd say the average was not more than ~5. The average
> _without_ deadlocks should be lower, if anything.

Right, I wasn't expecting the list to be large, certainly no larger than
cores typically. On the borderline of where a more complex tree starts
to pay off.

> I saw that your test cases go quite a bit higher, but even the
> rather extreme load I was testing with -- which is not quite a load
> from an actual application, though it is related to one -- has 40
> threads and so a theoretical maximum of 40.

The stress loads were just values plucked out of nowhere to try and have
a reasonable stab at hitting the deadlock. Certainly if we were to wrap
that up in a microbenchmark we would want to have wider coverage (so the
graph against contention is more useful).

Do you have a branch I can pull the patches for (or what did you use as
the base)?

Chris Wilson, Intel Open Source Technology Centre