From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756206Ab2IKGL7 (ORCPT ); Tue, 11 Sep 2012 02:11:59 -0400 Received: from e23smtp03.au.ibm.com ([202.81.31.145]:37926 "EHLO e23smtp03.au.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753939Ab2IKGL5 (ORCPT ); Tue, 11 Sep 2012 02:11:57 -0400 Message-ID: <504ED54E.6040608@linux.vnet.ibm.com> Date: Tue, 11 Sep 2012 11:38:14 +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: habanero@linux.vnet.ibm.com CC: Peter Zijlstra , Srikar Dronamraju , 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> <1347307972.7332.78.camel@oc2024037011.ibm.com> In-Reply-To: <1347307972.7332.78.camel@oc2024037011.ibm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit x-cbid: 12091106-6102-0000-0000-00000236828A Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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..