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    SubjectRe: [RFC][PATCH] Improving directed yield scalability for PLE handler
    On 09/11/2012 01:42 AM, Andrew Theurer wrote:
    > On Mon, 2012-09-10 at 19:12 +0200, Peter Zijlstra wrote:
    >> On Mon, 2012-09-10 at 22:26 +0530, Srikar Dronamraju wrote:
    >>>> +static bool __yield_to_candidate(struct task_struct *curr, struct task_struct *p)
    >>>> +{
    >>>> + if (!curr->sched_class->yield_to_task)
    >>>> + return false;
    >>>> +
    >>>> + if (curr->sched_class != p->sched_class)
    >>>> + return false;
    >>>
    >>>
    >>> Peter,
    >>>
    >>> Should we also add a check if the runq has a skip buddy (as pointed out
    >>> by Raghu) and return if the skip buddy is already set.
    >>
    >> Oh right, I missed that suggestion.. the performance improvement went
    >> from 81% to 139% using this, right?
    >>
    >> It might make more sense to keep that separate, outside of this
    >> function, since its not a strict prerequisite.
    >>
    >>>>
    >>>> + if (task_running(p_rq, p) || p->state)
    >>>> + return false;
    >>>> +
    >>>> + return true;
    >>>> +}
    >>
    >>
    >>>> @@ -4323,6 +4340,10 @@ bool __sched yield_to(struct task_struct *p,
    >>> bool preempt)
    >>>> rq = this_rq();
    >>>>
    >>>> again:
    >>>> + /* optimistic test to avoid taking locks */
    >>>> + if (!__yield_to_candidate(curr, p))
    >>>> + goto out_irq;
    >>>> +
    >>
    >> So add something like:
    >>
    >> /* Optimistic, if we 'raced' with another yield_to(), don't bother */
    >> if (p_rq->cfs_rq->skip)
    >> goto out_irq;
    >>>
    >>>
    >>>> p_rq = task_rq(p);
    >>>> double_rq_lock(rq, p_rq);
    >>>
    >>>
    >> But I do have a question on this optimization though,.. Why do we check
    >> p_rq->cfs_rq->skip and not rq->cfs_rq->skip ?
    >>
    >> That is, I'd like to see this thing explained a little better.
    >>
    >> Does it go something like: p_rq is the runqueue of the task we'd like to
    >> yield to, rq is our own, they might be the same. If we have a ->skip,
    >> there's nothing we can do about it, OTOH p_rq having a ->skip and
    >> failing the yield_to() simply means us picking the next VCPU thread,
    >> which might be running on an entirely different cpu (rq) and could
    >> succeed?
    >
    > Here's two new versions, both include a __yield_to_candidate(): "v3"
    > uses the check for p_rq->curr in guest mode, and "v4" uses the cfs_rq
    > skip check. Raghu, I am not sure if this is exactly what you want
    > implemented in v4.
    >

    Andrew, Yes that is what I had. I think there was a mis-understanding.
    My intention was to if there is a directed_yield happened in runqueue
    (say rqA), do not bother to directed yield to that. But unfortunately as
    PeterZ pointed that would have resulted in setting next buddy of a
    different run queue than rqA.
    So we can drop this "skip" idea. Pondering more over what to do? can we
    use next buddy itself ... thinking..



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