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    SubjectRe: [PATCH V3 RFC 1/2] sched: Bail out of yield_to when source and target runqueue has one task
    On Mon, Nov 26, 2012 at 05:37:54PM +0530, Raghavendra K T wrote:
    > From: Peter Zijlstra <peterz@infradead.org>
    >
    > In case of undercomitted scenarios, especially in large guests
    > yield_to overhead is significantly high. when run queue length of
    > source and target is one, take an opportunity to bail out and return
    > -ESRCH. This return condition can be further exploited to quickly come
    > out of PLE handler.
    >
    > (History: Raghavendra initially worked on break out of kvm ple handler upon
    > seeing source runqueue length = 1, but it had to export rq length).
    > Peter came up with the elegant idea of return -ESRCH in scheduler core.
    >
    > Signed-off-by: Peter Zijlstra <peterz@infradead.org>
    > Raghavendra, Checking the rq length of target vcpu condition added.(thanks Avi)
    > Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
    > Signed-off-by: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
    > ---
    >
    > kernel/sched/core.c | 25 +++++++++++++++++++------
    > 1 file changed, 19 insertions(+), 6 deletions(-)
    >
    > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    > index 2d8927f..fc219a5 100644
    > --- a/kernel/sched/core.c
    > +++ b/kernel/sched/core.c
    > @@ -4289,7 +4289,10 @@ EXPORT_SYMBOL(yield);
    > * It's the caller's job to ensure that the target task struct
    > * can't go away on us before we can do any checks.
    > *
    > - * Returns true if we indeed boosted the target task.
    > + * Returns:
    > + * true (>0) if we indeed boosted the target task.
    > + * false (0) if we failed to boost the target.
    > + * -ESRCH if there's no task to yield to.
    > */
    > bool __sched yield_to(struct task_struct *p, bool preempt)
    > {
    > @@ -4303,6 +4306,15 @@ bool __sched yield_to(struct task_struct *p, bool preempt)
    >
    > again:
    > p_rq = task_rq(p);
    > + /*
    > + * If we're the only runnable task on the rq and target rq also
    > + * has only one task, there's absolutely no point in yielding.
    > + */
    > + if (rq->nr_running == 1 && p_rq->nr_running == 1) {
    > + yielded = -ESRCH;
    > + goto out_irq;
    > + }
    > +
    > double_rq_lock(rq, p_rq);
    > while (task_rq(p) != p_rq) {
    > double_rq_unlock(rq, p_rq);
    > @@ -4310,13 +4322,13 @@ again:
    > }
    >
    > if (!curr->sched_class->yield_to_task)
    > - goto out;
    > + goto out_unlock;
    >
    > if (curr->sched_class != p->sched_class)
    > - goto out;
    > + goto out_unlock;
    >
    > if (task_running(p_rq, p) || p->state)
    > - goto out;
    > + goto out_unlock;
    >
    > yielded = curr->sched_class->yield_to_task(rq, p, preempt);
    > if (yielded) {
    > @@ -4329,11 +4341,12 @@ again:
    > resched_task(p_rq->curr);
    > }
    >
    > -out:
    > +out_unlock:
    > double_rq_unlock(rq, p_rq);
    > +out_irq:
    > local_irq_restore(flags);
    >
    > - if (yielded)
    > + if (yielded > 0)
    > schedule();
    >
    > return yielded;
    >

    Acked-by: Andrew Jones <drjones@redhat.com>


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