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

    --
    Chris Wilson, Intel Open Source Technology Centre

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