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

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

On 09/10/2012 10:42 PM, 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?
>

Yes, That is the intention (mean checking  p_rq->cfs->skip). Though we
may be overdoing this check in the scenario when multiple vcpus of same
VM pinned to same CPU.

I am testing the above patch. Hope to be able to get back with the
results tomorrow.