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    SubjectRe: [RFC][PATCH] Improving directed yield scalability for PLE handler
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    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.

    Results:
    > ple on: 2552 +/- .70%
    > ple on: w/fixv1: 4621 +/- 2.12% (81% improvement)
    > ple on: w/fixv2: 6115 (139% improvement)
    v3: 5735 (124% improvement)
    v4: 4524 ( 3% regression)

    Both patches included below

    -Andrew

    diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    index fbf1fd0..0d98a67 100644
    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -4820,6 +4820,23 @@ void __sched yield(void)
    }
    EXPORT_SYMBOL(yield);

    +/*
    + * Tests preconditions required for sched_class::yield_to().
    + */
    +static bool __yield_to_candidate(struct task_struct *curr, struct task_struct *p, struct rq *p_rq)
    +{
    + if (!curr->sched_class->yield_to_task)
    + return false;
    +
    + if (curr->sched_class != p->sched_class)
    + return false;
    +
    + if (task_running(p_rq, p) || p->state)
    + return false;
    +
    + return true;
    +}
    +
    /**
    * yield_to - yield the current processor to another thread in
    * your thread group, or accelerate that thread toward the
    @@ -4844,20 +4861,27 @@ bool __sched yield_to(struct task_struct *p, bool preempt)

    again:
    p_rq = task_rq(p);
    +
    + /* optimistic test to avoid taking locks */
    + if (!__yield_to_candidate(curr, p, p_rq))
    + goto out_irq;
    +
    + /*
    + * if the target task is not running, then only yield if the
    + * current task is in guest mode
    + */
    + if (!(p_rq->curr->flags & PF_VCPU))
    + goto out_irq;
    +
    double_rq_lock(rq, p_rq);
    while (task_rq(p) != p_rq) {
    double_rq_unlock(rq, p_rq);
    goto again;
    }

    - if (!curr->sched_class->yield_to_task)
    - goto out;
    -
    - if (curr->sched_class != p->sched_class)
    - goto out;
    -
    - if (task_running(p_rq, p) || p->state)
    - goto out;
    + /* validate state, holding p_rq ensures p's state cannot change */
    + if (!__yield_to_candidate(curr, p, p_rq))
    + goto out_unlock;

    yielded = curr->sched_class->yield_to_task(rq, p, preempt);
    if (yielded) {
    @@ -4877,8 +4901,9 @@ again:
    rq->skip_clock_update = 0;
    }

    -out:
    +out_unlock:
    double_rq_unlock(rq, p_rq);
    +out_irq:
    local_irq_restore(flags);

    if (yielded)


    ****************
    v4:

    diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    index fbf1fd0..2bec2ed 100644
    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -4820,6 +4820,23 @@ void __sched yield(void)
    }
    EXPORT_SYMBOL(yield);

    +/*
    + * Tests preconditions required for sched_class::yield_to().
    + */
    +static bool __yield_to_candidate(struct task_struct *curr, struct task_struct *p, struct rq *p_rq)
    +{
    + if (!curr->sched_class->yield_to_task)
    + return false;
    +
    + if (curr->sched_class != p->sched_class)
    + return false;
    +
    + if (task_running(p_rq, p) || p->state)
    + return false;
    +
    + return true;
    +}
    +
    /**
    * yield_to - yield the current processor to another thread in
    * your thread group, or accelerate that thread toward the
    @@ -4844,20 +4861,24 @@ bool __sched yield_to(struct task_struct *p, bool preempt)

    again:
    p_rq = task_rq(p);
    +
    + /* optimistic test to avoid taking locks */
    + if (!__yield_to_candidate(curr, p, p_rq))
    + goto out_irq;
    +
    + /* if a yield is in progress, skip */
    + if (p_rq->cfs.skip)
    + goto out_irq;
    +
    double_rq_lock(rq, p_rq);
    while (task_rq(p) != p_rq) {
    double_rq_unlock(rq, p_rq);
    goto again;
    }

    - if (!curr->sched_class->yield_to_task)
    - goto out;
    -
    - if (curr->sched_class != p->sched_class)
    - goto out;
    -
    - if (task_running(p_rq, p) || p->state)
    - goto out;
    + /* validate state, holding p_rq ensures p's state cannot change */
    + if (!__yield_to_candidate(curr, p, p_rq))
    + goto out_unlock;

    yielded = curr->sched_class->yield_to_task(rq, p, preempt);
    if (yielded) {
    @@ -4877,8 +4898,9 @@ again:
    rq->skip_clock_update = 0;
    }

    -out:
    +out_unlock:
    double_rq_unlock(rq, p_rq);
    +out_irq:
    local_irq_restore(flags);

    if (yielded)



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