From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758244Ab2IJTNm (ORCPT ); Mon, 10 Sep 2012 15:13:42 -0400 Received: from e23smtp02.au.ibm.com ([202.81.31.144]:55762 "EHLO e23smtp02.au.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758189Ab2IJTNl (ORCPT ); Mon, 10 Sep 2012 15:13:41 -0400 Message-ID: <504E3B0F.6060108@linux.vnet.ibm.com> Date: Tue, 11 Sep 2012 00:40:07 +0530 From: Raghavendra K T Organization: IBM User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.1) Gecko/20120216 Thunderbird/10.0.1 MIME-Version: 1.0 To: Peter Zijlstra CC: Srikar Dronamraju , habanero@linux.vnet.ibm.com, Avi Kivity , Marcelo Tosatti , Ingo Molnar , Rik van Riel , KVM , chegu vinod , LKML , X86 , Gleb Natapov , Srivatsa Vaddagiri Subject: Re: [RFC][PATCH] Improving directed yield scalability for PLE handler 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> In-Reply-To: <1347297124.2124.42.camel@twins> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit x-cbid: 12091019-5490-0000-0000-0000021C8DEB Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@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.