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

[Nicolai HÃhnle]

On 30.11.2016 13:20, Chris Wilson wrote:
> 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)?

See git://people.freedesktop.org/~nh/linux mutex or 
https://cgit.freedesktop.org/~nh/linux/log/?h=mutex.

It's based on tip/core/locking + agd5f's drm-next, the latter only 
because I needed it for the test application.

Nicolai