From mboxrd@z Thu Jan  1 00:00:00 1970
From: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Subject: Re: [RFC][PATCH] Improving directed yield scalability for PLE handler
Date: Tue, 11 Sep 2012 00:40:07 +0530
Message-ID: <504E3B0F.6060108@linux.vnet.ibm.com>
References: <20120718133717.5321.71347.sendpatchset@codeblue.in.ibm.com> <500D2162.8010209@redhat.com> <1347023509.10325.53.camel@oc6622382223.ibm.com> <504A37B0.7020605@linux.vnet.ibm.com> <1347046931.7332.51.camel@oc2024037011.ibm.com> <20120908084345.GU30238@linux.vnet.ibm.com> <1347283005.10325.55.camel@oc6622382223.ibm.com> <1347293035.2124.22.camel@twins>	 <20120910165653.GA28033@linux.vnet.ibm.com> <1347297124.2124.42.camel@twins>
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Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
	habanero@linux.vnet.ibm.com, Avi Kivity <avi@redhat.com>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	Ingo Molnar <mingo@redhat.com>, Rik van Riel <riel@redhat.com>,
	KVM <kvm@vger.kernel.org>, chegu vinod <chegu_vinod@hp.com>,
	LKML <linux-kernel@vger.kernel.org>, X86 <x86@kernel.org>,
	Gleb Natapov <gleb@redhat.com>,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <1347297124.2124.42.camel@twins>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: kvm.vger.kernel.org

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.