From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751646AbcFVIYE (ORCPT ); Wed, 22 Jun 2016 04:24:04 -0400 Received: from merlin.infradead.org ([205.233.59.134]:47067 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751549AbcFVIX7 (ORCPT ); Wed, 22 Jun 2016 04:23:59 -0400 Date: Wed, 22 Jun 2016 10:23:57 +0200 From: Peter Zijlstra To: Konstantin Khlebnikov Cc: Ingo Molnar , linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH] sched/fair: initialize throttle_count for new task-groups lazily Message-ID: <20160622082357.GA30154@twins.programming.kicks-ass.net> References: <146608182119.21870.8439834428248129633.stgit@buzz> <20160621211054.GV30154@twins.programming.kicks-ass.net> <576A4806.3090701@yandex-team.ru> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <576A4806.3090701@yandex-team.ru> User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jun 22, 2016 at 11:10:46AM +0300, Konstantin Khlebnikov wrote: > On 22.06.2016 00:10, Peter Zijlstra wrote: > >On Thu, Jun 16, 2016 at 03:57:01PM +0300, Konstantin Khlebnikov wrote: > >>Cgroup created inside throttled group must inherit current throttle_count. > >>Broken throttle_count allows to nominate throttled entries as a next buddy, > >>later this leads to null pointer dereference in pick_next_task_fair(). > >> > >>This patch initialize cfs_rq->throttle_count at first enqueue: laziness > >>allows to skip locking all rq at group creation. Lazy approach also allows > >>to skip full sub-tree scan at throttling hierarchy (not in this patch). > > > >You're talking about taking rq->lock in alloc_fair_sched_group(), right? > > > >We're about to go do that anyway... But I suppose for backports this > >makes sense. Doing it at creation time also avoids the issues Ben > >raised, right? > > Yes, all will be fine. But for 8192-cores this will be disaster =) Well, creating cgroups isn't something you do much of, and creating them will be proportionally expensive already, as we allocate all kinds of per-cpu data. In any case, we 'need' to do this because of the per entity load tracking stuff, entities, even blocked, should be added to the cfs_rq. > throttle_count must be initialized after linking tg into lists. obviously. Crud, that's later than we currently take the rq lock. Let me see how much pain it is to re-order all that.