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SubjectRe: [PATCH V3 RFC 1/2] sched: Bail out of yield_to when source and target runqueue has one task
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;
>>
>> --
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>
>
>



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