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

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Message-ID: <1347367710.7332.81.camel@oc2024037011.ibm.com>
Date:	Tue, 11 Sep 2012 07:48:30 -0500
From:	Andrew Theurer <habanero@...ux.vnet.ibm.com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Ingo Molnar <mingo@...hat.com>, Rik van Riel <riel@...hat.com>,
	KVM <kvm@...r.kernel.org>, chegu vinod <chegu_vinod@...com>,
	LKML <linux-kernel@...r.kernel.org>, X86 <x86@...nel.org>,
	Gleb Natapov <gleb@...hat.com>,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>
Subject: Re: [RFC][PATCH] Improving directed yield scalability for PLE
 handler

On Tue, 2012-09-11 at 11:38 +0530, Raghavendra K T wrote:
> 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..

FYI, I regretfully forgot include your recent changes to
kvm_vcpu_on_spin in my tests (found in kvm.git/next branch), so I am
going to get some results for that before I experiment any more on
3.6-rc.

-Andrew


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