Messages in this thread | | | Date | Tue, 11 Sep 2012 11:38:14 +0530 | From | Raghavendra K T <> | Subject | Re: [RFC][PATCH] Improving directed yield scalability for PLE handler |
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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..
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