From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934035AbcLPI4U (ORCPT ); Fri, 16 Dec 2016 03:56:20 -0500 Received: from mail-oi0-f47.google.com ([209.85.218.47]:33087 "EHLO mail-oi0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757181AbcLPI4M (ORCPT ); Fri, 16 Dec 2016 03:56:12 -0500 MIME-Version: 1.0 In-Reply-To: <20161215214214.GC3124@twins.programming.kicks-ass.net> References: <1480610333-23329-1-git-send-email-vincent.guittot@linaro.org> <20161215214214.GC3124@twins.programming.kicks-ass.net> From: Vincent Guittot Date: Fri, 16 Dec 2016 09:55:51 +0100 Message-ID: Subject: Re: [PATCH] sched: fix group_entity's share update To: Peter Zijlstra Cc: Ingo Molnar , linux-kernel , Paul Turner , stable@vger.kernel.org Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 15 December 2016 at 22:42, Peter Zijlstra wrote: > > On Thu, Dec 01, 2016 at 05:38:53PM +0100, Vincent Guittot wrote: > > The update of the share of a cfs_rq is done when its load_avg is updated > > but before the group_entity's load_avg has been updated for the past time > > slot. This generates wrong load_avg accounting which can be significant > > when small tasks are involved in the scheduling. > > > > Let take the example of a task TA that is dequeued of its task group TG1. > > TA was the only task in TG1 which becomes idle. > > > > We have the sequence: > > > > - dequeue_entity TA->se > > - update_load_avg(TA->se) > > - dequeue_entity_load_avg(TG1->cfs_rq, TA->se) > > - account_entity_dequeue(TG1->cfs_rq, TA->se) > > TG1->cfs_rq->load.weight = 0 > > - update_cfs_shares(TG1->cfs_rq) > > TG1->se->load.weight is updated with the new share of > > cfs_rq. TG1->se->load.weight = 0. > > - dequeue_entity TG1->se > > - update_load_avg(TG1->se) but its weight is now null so the last time > > slot (up to a tick) will be accounted with its new weight (0 in our case) > > instead of its real weight. The last time slot is accounted as an idle one > > whereas it was a running one. > > > > If the running time of TA is short enough that no tick happens when it > > runs, all running time of TG1->se will be accounted as idle time. > > > > Instead, we should update the share of a cfs_rq (in fact the weight of its > > group entity) only after having updated the load_avg of the group_entity. > > > > update_cfs_shares() now takes the sched_entity as parameter instead of the > > cfs_rq and the weight of the group_entity is updated only once its load_avg > > has been synced with current time. > > Urgh, brain hurt, also those names don't help; s/TG1/A/ s/TA/a/ > > So the problem is that in our for_each_sched_entity(se) loop we end up > changing the next se before we get there. > > > root > (cfs_rq) > \ > (se) > A > (cfs_rq) > \ > (se) > a > > > Starting at a's se, we update_cfs_shares() on A's cfs_rq, which then > updates A's se, which is the next se in our iteration and mucks with > state before we get there. > > So you change update_cfs_shares() to go downward while we go upward, > ensuring we only update things that we've finished with. yes > > Makes sense.. > > > kernel/sched/fair.c | 27 ++++++++++++++++----------- > > 1 file changed, 16 insertions(+), 11 deletions(-) > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index 18d9e75..19092fa 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -2689,15 +2689,18 @@ static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, > > > > static inline int throttled_hierarchy(struct cfs_rq *cfs_rq); > > > > -static void update_cfs_shares(struct cfs_rq *cfs_rq) > > +static void update_cfs_shares(struct sched_entity *se) > > { > > struct task_group *tg; > > - struct sched_entity *se; > > + struct cfs_rq *cfs_rq = group_cfs_rq(se); > > long shares; > > please keep them ordered by length. Ok > > > > > + if (entity_is_task(se)) > > can be: !cfs_rq, which is the same and we already done that load. yes. My goal was to keep it more readable about the meaning of the test and I was expecting that the compiler would be smart enough to use the same one load for both cfs_rq = group_cfs_rq(se) and entity_is_task(se) I can change with !cfs_rq > > > + return; > > + > > tg = cfs_rq->tg; > > This load isn't needed here yet, can be moved down a bit. Indeed > > > - se = tg->se[cpu_of(rq_of(cfs_rq))]; > > - if (!se || throttled_hierarchy(cfs_rq)) > > + > > + if (throttled_hierarchy(cfs_rq)) > > return; > > #ifndef CONFIG_SMP > > if (likely(se->load.weight == tg->shares)) > > > > @@ -3583,9 +3588,9 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags) > > se->vruntime += cfs_rq->min_vruntime; > > > > update_load_avg(se, UPDATE_TG); > > + update_cfs_shares(se); > > enqueue_entity_load_avg(cfs_rq, se); > > account_entity_enqueue(cfs_rq, se); > > - update_cfs_shares(cfs_rq); > > > > if (flags & ENQUEUE_WAKEUP) > > place_entity(cfs_rq, se, 0); > > So here we need to update_cfs_shares() _before_ enqueue_entity, because > the update_cfs_shares() will affect this se's load, right? exactly > > > @@ -3681,7 +3686,7 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags) > > /* return excess runtime on last dequeue */ > > return_cfs_rq_runtime(cfs_rq); > > > > - update_cfs_shares(cfs_rq); > > + update_cfs_shares(se); > > > > /* > > * Now advance min_vruntime if @se was the entity holding it back, > > But this one hurts my brain.. > > It must be done after dequeue_entity_load_avg() such that we subtract > the load as was seen until now. update_cfs_shares(A's se) must be done after update_load_avg(A's se, UPDATE_TG); so the update od A's se ->load-avg will be updated with the previous load to update load_avg for the previous time slot. update_cfs_shares(A's se) could be done before or after dequeue_entity_load_avg(A's se) because the root's cfs_rq is kept sync during the reweight of A's se. Nevertheless, i see one advantage of doing that after: reweight_entity will be faster because A's se->on_rq will have been cleared in the meantime > > Could we please add comments explaining this ordering, because I forever > need to think about this (both enqueue and dequeue). OK > > > @@ -3864,7 +3869,7 @@ entity_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr, int queued) > > * Ensure that runnable average is periodically updated. > > */ > > update_load_avg(curr, UPDATE_TG); > > - update_cfs_shares(cfs_rq); > > + update_cfs_shares(curr); > > > > #ifdef CONFIG_SCHED_HRTICK > > /* > > @@ -4761,7 +4766,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags) > > break; > > > > update_load_avg(se, UPDATE_TG); > > - update_cfs_shares(cfs_rq); > > + update_cfs_shares(se); > > } > > > > if (!se) > > @@ -4820,7 +4825,7 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags) > > break; > > > > update_load_avg(se, UPDATE_TG); > > - update_cfs_shares(cfs_rq); > > + update_cfs_shares(se); > > } > > > > if (!se) > > This has a distinct pattern to it though; should we think about > something like: UPDATE_SHARES for update_load_avg() or does that confuse > things? IMHO, keeping update_cfs_shares separated from update_load_avg make it clear about when we update the shares and enable some optimization like for dequeue_entity > > > @@ -9316,7 +9321,7 @@ int sched_group_set_shares(struct task_group *tg, unsigned long shares) > > /* Possible calls to update_curr() need rq clock */ > > update_rq_clock(rq); > > for_each_sched_entity(se) > > - update_cfs_shares(group_cfs_rq(se)); > > + update_cfs_shares(se); > > Should we not also catch up with our load before we frob the shares? yes you're right, an update_load_avg is missing > > > raw_spin_unlock_irqrestore(&rq->lock, flags); > > }