Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755383AbbGQRC3 (ORCPT ); Fri, 17 Jul 2015 13:02:29 -0400 Received: from mail-pa0-f46.google.com ([209.85.220.46]:35464 "EHLO mail-pa0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754837AbbGQRC0 (ORCPT ); Fri, 17 Jul 2015 13:02:26 -0400 From: bsegall@google.com To: Byungchul Park Cc: pjt@google.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH] sched: prevent sched entity from being decayed twice when both waking and migrating it References: <1437034317-15120-1-git-send-email-byungchul.park@lge.com> <20150717061949.GD3956@byungchulpark-X58A-UD3R> Date: Fri, 17 Jul 2015 10:02:22 -0700 In-Reply-To: <20150717061949.GD3956@byungchulpark-X58A-UD3R> (Byungchul Park's message of "Fri, 17 Jul 2015 15:19:49 +0900") Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5199 Lines: 130 Byungchul Park writes: > On Thu, Jul 16, 2015 at 10:00:00AM -0700, bsegall@google.com wrote: > > hello, > >> byungchul.park@lge.com writes: >> >> > From: Byungchul Park >> > >> > hello paul, >> > >> > can i ask you something? >> > >> > when a sched entity is both waken and migrated, it looks being decayed twice. >> > did you do it on purpose? >> > or am i missing something? :( >> > >> > thanks, >> > byungchul >> >> __synchronize_entity_decay() updates only se->avg.load_avg_contrib so >> that removing from blocked_load is done correctly. > > as you said, it should done here. :) > >> update_entity_load_avg() accounts that (approximation of) time blocked > > i mean the entity was already accounted the blocked time in > __synchronize_entity_decay(). > >> against runnable_avg/running_avg (and then recomputes load_avg_contrib >> to match while load_avg_contrib isn't part of any cfs_rq's sum). > > the thing to keep in mind is that, currently load tracking is done by > per-entity. that is, the entity already has its own whole load_avg_contrib > with considering the entity's blocked time, after __synchronize_entity_decay(). > and cfs_rq can account the se's load by adding se->avg.load_avg_contrib to > cfs_rq->runnable_load_avg, like enqueue_entity_load_avg() code. > > wrong? load_avg_contrib is computed from runnable_avg, which is not updated by __synchronize_entity_decay, only by update_entity_load_avg -> __update_entity_runnable_avg. __synchronize_entity_decay is used in this path because update_entity_load_avg needs the rq lock (along with some other reasons), and migrate_task_rq_fair generally doesn't have the lock. > > thanks, > byungchul > >> >> > >> > --------------->8--------------- >> > From 793c963d0b29977a0f6f9330291a9ea469cc54f0 Mon Sep 17 00:00:00 2001 >> > From: Byungchul Park >> > Date: Thu, 16 Jul 2015 16:49:48 +0900 >> > Subject: [PATCH] sched: prevent sched entity from being decayed twice when >> > both waking and migrating it >> > >> > current code is decaying load average variables with a sleep time twice, >> > when both waking and migrating it. the first decaying happens in a call path >> > "migrate_task_rq_fair() -> __synchronize_entity_decay()". the second >> > decaying happens in a call path "enqueue_entity_load_avg() -> >> > update_entity_load_avg()". so make it happen once. >> > >> > Signed-off-by: Byungchul Park >> > --- >> > kernel/sched/fair.c | 29 +++-------------------------- >> > 1 file changed, 3 insertions(+), 26 deletions(-) >> > >> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >> > index 09456fc..c86cca0 100644 >> > --- a/kernel/sched/fair.c >> > +++ b/kernel/sched/fair.c >> > @@ -2873,32 +2873,9 @@ static inline void enqueue_entity_load_avg(struct cfs_rq *cfs_rq, >> > struct sched_entity *se, >> > int wakeup) >> > { >> > - /* >> > - * We track migrations using entity decay_count <= 0, on a wake-up >> > - * migration we use a negative decay count to track the remote decays >> > - * accumulated while sleeping. >> > - * >> > - * Newly forked tasks are enqueued with se->avg.decay_count == 0, they >> > - * are seen by enqueue_entity_load_avg() as a migration with an already >> > - * constructed load_avg_contrib. >> > - */ >> > - if (unlikely(se->avg.decay_count <= 0)) { >> > + /* we track migrations using entity decay_count == 0 */ >> > + if (unlikely(!se->avg.decay_count)) { >> > se->avg.last_runnable_update = rq_clock_task(rq_of(cfs_rq)); >> > - if (se->avg.decay_count) { >> > - /* >> > - * In a wake-up migration we have to approximate the >> > - * time sleeping. This is because we can't synchronize >> > - * clock_task between the two cpus, and it is not >> > - * guaranteed to be read-safe. Instead, we can >> > - * approximate this using our carried decays, which are >> > - * explicitly atomically readable. >> > - */ >> > - se->avg.last_runnable_update -= (-se->avg.decay_count) >> > - << 20; >> > - update_entity_load_avg(se, 0); >> > - /* Indicate that we're now synchronized and on-rq */ >> > - se->avg.decay_count = 0; >> > - } >> > wakeup = 0; >> > } else { >> > __synchronize_entity_decay(se); >> > @@ -5114,7 +5091,7 @@ migrate_task_rq_fair(struct task_struct *p, int next_cpu) >> > * be negative here since on-rq tasks have decay-count == 0. >> > */ >> > if (se->avg.decay_count) { >> > - se->avg.decay_count = -__synchronize_entity_decay(se); >> > + __synchronize_entity_decay(se); >> > atomic_long_add(se->avg.load_avg_contrib, >> > &cfs_rq->removed_load); >> > } >> -- >> 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/ -- 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/