linux-kernel - Re: [PATCH V3 RFC 1/2] sched: Bail out of yield_to when source and target runqueue has one task

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Date:	Tue, 27 Nov 2012 06:23:04 -0800
From:	Chegu Vinod <chegu_vinod@...com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
CC:	Andrew Jones <drjones@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	"Andrew M. Theurer" <habanero@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Avi Kivity <avi@...hat.com>, Rik van Riel <riel@...hat.com>,
	Srikar <srikar@...ux.vnet.ibm.com>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	KVM <kvm@...r.kernel.org>, Jiannan Ouyang <ouyang@...pitt.edu>,
	LKML <linux-kernel@...r.kernel.org>,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>
Subject: Re: [PATCH V3 RFC 1/2] sched: Bail out of yield_to when source and
 target runqueue has one task

On 11/27/2012 2:30 AM, Raghavendra K T wrote:
> On 11/26/2012 07:05 PM, Andrew Jones wrote:
>> On Mon, Nov 26, 2012 at 05:37:54PM +0530, Raghavendra K T wrote:
>>> From: Peter Zijlstra <peterz@...radead.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@...radead.org>
>>> Raghavendra, Checking the rq length of target vcpu condition 
>>> added.(thanks Avi)
>>> Reviewed-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
>>> Signed-off-by: Raghavendra K T <raghavendra.kt@...ux.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@...hat.com>
>>
>
> Thank you Drew.
>
> Marcelo Gleb.. Please let me know if you have comments / concerns on 
> the patches..
>
> Andrew, Vinod, IMO, the patch set looks good for undercommit scenarios
> especially for large guests where we do have overhead of vcpu iteration
> of ple handler..
>
> .
>
Thanks Raghu. Will try to get this latest patch set evaluated and get 
back to you.

Vinod

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