From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S964786AbcE0UjT (ORCPT ); Fri, 27 May 2016 16:39:19 -0400 Received: from foss.arm.com ([217.140.101.70]:46305 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933141AbcE0UjD (ORCPT ); Fri, 27 May 2016 16:39:03 -0400 Subject: Re: [PATCH v2] sched: fix first task of a task group is attached twice To: Vincent Guittot References: <1464095335-27730-1-git-send-email-vincent.guittot@linaro.org> <1464188472-30086-1-git-send-email-vincent.guittot@linaro.org> <57486C62.7090205@arm.com> Cc: Peter Zijlstra , Ingo Molnar , linux-kernel , Yuyang Du From: Dietmar Eggemann Message-ID: <5748B063.4070009@arm.com> Date: Fri, 27 May 2016 21:38:59 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 27/05/16 18:16, Vincent Guittot wrote: > On 27 May 2016 at 17:48, Dietmar Eggemann wrote: >> On 25/05/16 16:01, Vincent Guittot wrote: >>> The cfs_rq->avg.last_update_time is initialize to 0 with the main effect >>> that the 1st sched_entity that will be attached, will keep its >>> last_update_time set to 0 and will attached once again during the >>> enqueue. >>> Initialize cfs_rq->avg.last_update_time to 1 instead. >>> >>> Signed-off-by: Vincent Guittot >>> --- >>> >>> v2: >>> - rq_clock_task(rq_of(cfs_rq)) can't be used because lock is not held >>> >>> kernel/sched/fair.c | 8 ++++++++ >>> 1 file changed, 8 insertions(+) >>> >>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >>> index 218f8e8..3724656 100644 >>> --- a/kernel/sched/fair.c >>> +++ b/kernel/sched/fair.c >>> @@ -8586,6 +8586,14 @@ void init_tg_cfs_entry(struct task_group *tg, struct cfs_rq *cfs_rq, >>> se->depth = parent->depth + 1; >>> } >>> >>> + /* >>> + * Set last_update_time to something different from 0 to make >>> + * sure the 1st sched_entity will not be attached twice: once >>> + * when attaching the task to the group and one more time when >>> + * enqueueing the task. >>> + */ >>> + tg->cfs_rq[cpu]->avg.last_update_time = 1; >>> + Couldn't you not just set the value in init_cfs_rq(): @@ -8482,6 +8482,7 @@ void init_cfs_rq(struct cfs_rq *cfs_rq) cfs_rq->min_vruntime_copy = cfs_rq->min_vruntime; #endif #ifdef CONFIG_SMP + cfs_rq->avg.last_update_time = 1; atomic_long_set(&cfs_rq->removed_load_avg, 0); atomic_long_set(&cfs_rq->removed_util_avg, 0); #endif >>> se->my_q = cfs_rq; >>> /* guarantee group entities always have weight */ >>> update_load_set(&se->load, NICE_0_LOAD); >> >> So why not setting the last_update_time value for those cfs_rq's when >> we have the lock? E.g. in task_move_group_fair() or attach_task_cfs_rq(). > > I'm not sure that it's worth adding this init in functions that are > then used often only for the init of it. Yeah, there will be this if condition overhead. > If you are concerned by the update of the load of the 1st task that > will be attached, it can still have elapsed a long time between the > creation of the group and the 1st enqueue of a task. This was the case > for the test i did when i found this issue. Understood, but for me, creation of the task group is cpu_cgroup_css_alloc -> sched_create_group() -> ... -> init_cfs_rq(), init_tg_cfs_entry(), ... and the functions which are called when the first task is put into the task group are cpu_cgroup_attach() and cpu_cgroup_fork() and they whould trigger the initial setup of the cfs_rq->avg.last_update_time. > > Beside this point, I have to send a new version to set > load_last_update_time_copy for not 64 bits system. Fengguang points me > the issue OK. [...] >> >> + if (!cfs_rq->avg.last_update_time) >> + cfs_rq->avg.last_update_time = rq_clock_task(rq_of(cfs_rq)); >> + >> /* Synchronize task with its cfs_rq */ >> attach_entity_load_avg(cfs_rq, se); >