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

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

On 09/11/2012 01:42 AM, Andrew Theurer wrote:
> On Mon, 2012-09-10 at 19:12 +0200, 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?
>
> Here's two new versions, both include a __yield_to_candidate(): "v3"
> uses the check for p_rq->curr in guest mode, and "v4" uses the cfs_rq
> skip check.  Raghu, I am not sure if this is exactly what you want
> implemented in v4.
>

Andrew, Yes that is what I had. I think there was a mis-understanding. 
My intention was to if there is a directed_yield happened in runqueue 
(say rqA), do not bother to directed yield to that. But unfortunately as 
PeterZ pointed that would have resulted in setting next buddy of a 
different run queue than rqA.
So we can drop this "skip" idea. Pondering more over what to do? can we 
use next buddy itself ... thinking..