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    SubjectRe: [PATCH V3 RFC 1/2] sched: Bail out of yield_to when source and target runqueue has one task
    [I forgot to do TO to Ingo last time]

    Ingo,
    Could you please take this into x86 tree.
    This is
    Acked-by: Andrew Jones <drjones@redhat.com>
    Tested-by: Chegu Vinod <chegu_vinod@hp.com>

    Marcelo, do you want to add your Acked-by/Reviewed-by?

    On 12/14/2012 09:10 PM, Raghavendra K T wrote:
    > Hi Ingo,
    >
    > Could you please take this into x86 tree?
    >
    > Thanks,
    > On 12/14/2012 05:59 AM, Marcelo Tosatti wrote:
    >> Raghavendra,
    >>
    >> Please get this integrate through x86 tree (Ingo CC'ed).
    >>
    >> 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;
    >>>
    >>> --
    >>> To unsubscribe from this list: send the line "unsubscribe kvm" in
    >>> the body of a message to majordomo@vger.kernel.org
    >>> More majordomo info at http://vger.kernel.org/majordomo-info.html
    >>
    >>
    >>
    >



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