Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752444Ab1B1Dug (ORCPT ); Sun, 27 Feb 2011 22:50:36 -0500 Received: from e23smtp04.au.ibm.com ([202.81.31.146]:39014 "EHLO e23smtp04.au.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752410Ab1B1Due (ORCPT ); Sun, 27 Feb 2011 22:50:34 -0500 Date: Mon, 28 Feb 2011 09:20:46 +0530 From: Bharata B Rao To: Paul Turner Cc: Peter Zijlstra , linux-kernel@vger.kernel.org, Dhaval Giani , Balbir Singh , Vaidyanathan Srinivasan , Gautham R Shenoy , Srivatsa Vaddagiri , Kamalesh Babulal , Ingo Molnar , Pavel Emelyanov , Herbert Poetzl , Avi Kivity , Chris Friesen , Nikhil Rao Subject: Re: [CFS Bandwidth Control v4 3/7] sched: throttle cfs_rq entities which exceed their local quota Message-ID: <20110228035046.GB3005@in.ibm.com> Reply-To: bharata@linux.vnet.ibm.com References: <20110216031831.571628191@google.com> <20110216031841.068673650@google.com> <1298467933.2217.765.camel@twins> <20110225135856.GA2376@in.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3537 Lines: 90 On Fri, Feb 25, 2011 at 12:51:01PM -0800, Paul Turner wrote: > On Fri, Feb 25, 2011 at 5:58 AM, Bharata B Rao > wrote: > > On Thu, Feb 24, 2011 at 07:10:58PM -0800, Paul Turner wrote: > >> On Wed, Feb 23, 2011 at 5:32 AM, Peter Zijlstra wrote: > >> > On Tue, 2011-02-15 at 19:18 -0800, Paul Turner wrote: > >> > >> >> + ? ? update_cfs_load(cfs_rq, 0); > >> >> + > >> >> + ? ? /* prevent previous buddy nominations from re-picking this se */ > >> >> + ? ? clear_buddies(cfs_rq_of(se), se); > >> >> + > >> >> + ? ? /* > >> >> + ? ? ?* It's possible for the current task to block and re-wake before task > >> >> + ? ? ?* switch, leading to a throttle within enqueue_task->update_curr() > >> >> + ? ? ?* versus an an entity that has not technically been enqueued yet. > >> > > >> > I'm not quite seeing how this would happen.. care to expand on this? > >> > > >> > >> I'm not sure the example Bharata gave is correct -- I'm going to treat > >> that discussion separately as it's not the intent here. > > > > Just for the record, my examples were not given for the above question from > > Peter. > > > > I answered two questions and I am tempted to stand by those until proven > > wrong :) > > This is important to get right, I'm happy to elaborate. > > > > > 1. Why do we have cfs_rq_throtted() check in dequeue_task_fair() ? > > The check is primarily needed because we could become throttled as > part of a regular dequeue. At which point we bail because the parent > dequeue is actually complete. > > (Were it necessitated by load balance we could actually not do this > and just perform a hierarchal check within load_balance_fair) > > > ( => How could we be running if our parent was throttled ?) > > > > The only way we can be running if our parent was throttled is if /we/ > triggered that throttle and have been marked for re-schedule. > > > Consider the following hierarchy. > > > > Root Group > > ? | > > ? | > > Group 1 (Bandwidth constrained group) > > ? | > > ? | > > Group 2 (Infinite runtime group) > > > > Assume both the groups have tasks in them. > > > > When Group 1 is throttled, its cfs_rq is marked throttled, and is removed from > > Root group's runqueue. But leaf tasks in Group 2 continue to be enqueued in > > Group 1's runqueue. > > > > Yes, the hierarchy state is maintained in isolation. > > > Load balancer kicks in on CPU A and figures out that it can pull a few tasks > > from CPU B (busiest_cpu). It iterates through all the task groups > > (load_balance_fair) and considers Group 2 also. It tries to pull a task from > > CPU B's cfs_rq for Group 2. I don't see anything that would prevent the > > load balancer from bailing out here. > > Per above, the descendants of a throttled group are also identified > (and appropriately skipped) using h_load. This bit is still unclear to me. We do nothing in tg_load_down() to treat throttled cfs_rqs differently when calculating h_load. Nor do we do anything in load_balance_fair() to explicitly identify descendents of throttled group using h_load AFAICS. All we have is cfs_rq_throttled() check, which I think should be converted to entity_on_rq() to check for the throttled hierarchy and discard pulling from throttled hierarchies. Regards, Bharata. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/