From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 16DEBC433EF for ; Tue, 18 Jan 2022 10:56:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238235AbiARK4q (ORCPT ); Tue, 18 Jan 2022 05:56:46 -0500 Received: from foss.arm.com ([217.140.110.172]:53532 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S238144AbiARK4m (ORCPT ); Tue, 18 Jan 2022 05:56:42 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id C5EC5D6E; Tue, 18 Jan 2022 02:56:41 -0800 (PST) Received: from FVFF7649Q05P (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 2DE673F774; Tue, 18 Jan 2022 02:56:40 -0800 (PST) Date: Tue, 18 Jan 2022 10:56:37 +0000 From: Vincent Donnefort To: Vincent Guittot Cc: peterz@infradead.org, mingo@redhat.com, linux-kernel@vger.kernel.org, dietmar.eggemann@arm.com, Valentin.Schneider@arm.com, Morten.Rasmussen@arm.com, Chris.Redpath@arm.com, qperret@google.com, Lukasz.Luba@arm.com Subject: Re: [PATCH v2 2/7] sched/fair: Decay task PELT values during migration Message-ID: References: <20220112161230.836326-1-vincent.donnefort@arm.com> <20220112161230.836326-3-vincent.donnefort@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 17, 2022 at 06:31:25PM +0100, Vincent Guittot wrote: > On Wed, 12 Jan 2022 at 17:14, Vincent Donnefort > wrote: > > > > Before being migrated to a new CPU, a task sees its PELT values > > synchronized with rq last_update_time. Once done, that same task will also > > have its sched_avg last_update_time reset. This means the time between > > the migration and the last clock update (B) will not be accounted for in > > util_avg and a discontinuity will appear. This issue is amplified by the > > PELT clock scaling. If the clock hasn't been updated while the CPU is > > idle, clock_pelt will not be aligned with clock_task and that time (A) > > will be also lost. > > > > ---------|----- A -----|-----------|------- B -----|> > > clock_pelt clock_task clock now > > > > This is especially problematic for asymmetric CPU capacity systems which > > need stable util_avg signals for task placement and energy estimation. > > > > Ideally, this problem would be solved by updating the runqueue clocks > > before the migration. But that would require taking the runqueue lock > > which is quite expensive [1]. Instead estimate the missing time and update > > the task util_avg with that value: > > > > A + B = clock_task - clock_pelt + sched_clock_cpu() - clock > > > > Neither clock_task, clock_pelt nor clock can be accessed without the > > runqueue lock. The new runqueue clock_pelt_lag is therefore created and > > encode those three values. > > > > clock_pelt_lag = clock - clock_task + clock_pelt > > > > And we can then write the missing time as follow: > > > > A + B = sched_clock_cpu() - clock_pelt_lag > > > > The B. part of the missing time is however an estimation that doesn't take > > into account IRQ and Paravirt time. > > > > Now we have an estimation for A + B, we can create an estimator for the > > PELT value at the time of the migration. We need for this purpose to > > inject last_update_time which is a combination of both clock_pelt and > > lost_idle_time. The latter is a time value which is completely lost form a > > PELT point of view and must be ignored. And finally, we can write: > > > > rq_clock_pelt_estimator() = last_update_time + A + B > > = last_update_time + > > sched_clock_cpu() - clock_pelt_lag > > > > [1] https://lore.kernel.org/all/20190709115759.10451-1-chris.redpath@arm.com/ > > > > Signed-off-by: Vincent Donnefort > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > index 06cf7620839a..11c6aeef4583 100644 > > --- a/kernel/sched/core.c > > +++ b/kernel/sched/core.c > > @@ -618,6 +618,12 @@ struct rq *task_rq_lock(struct task_struct *p, struct rq_flags *rf) > > } > > } > > > > +static void update_rq_clock_pelt_lag(struct rq *rq) > > +{ > > + u64_u32_store(rq->clock_pelt_lag, > > + rq->clock - rq->clock_task + rq->clock_pelt); > > This has several shortfalls: > - have a look at cfs_rq_clock_pelt() and rq_clock_pelt(). What you > name clock_pelt in your commit message and is used to update PELT and > saved in se->avg.last_update_time is : rq->clock_pelt - > rq->lost_idle_time - cfs_rq->throttled_clock_task_time That's why, the PELT "lag" is added onto se->avg.last_update_time. (see the last paragraph of the commit message) The estimator is just a time delta, that is added on top of the entity's last_update_time. I don't see any problem with the lost_idle_time here. > - you are doing this whatever the state of the cpu : idle or not. But > the clock cycles are not accounted for in the same way in both cases. If the CPU is idle and clock_pelt == clock_task, the component A of the estimator would be 0 and we only would account for how outdated is the rq's clock, i.e. component B. > - (B) doesn't seem to be accurate as you skip irq and steal time > accounting and you don't apply any scale invariance if the cpu is not > idle The missing irq and paravirt time is the reason why it is called "estimator". But maybe there's a chance of improving this part with a lockless version of rq->prev_irq_time and rq->prev_steal_time_rq? > - IIUC your explanation in the commit message above, the (A) period > seems to be a problem only when idle but you apply it unconditionally. If the CPU is idle (and clock_pelt == clock_task), only the B part would be worth something: A + B = [clock_task - clock_pelt] + [sched_clock_cpu() - clock] A B > If cpu is idle you can assume that clock_pelt should be equal to > clock_task but you can't if cpu is not idle otherwise your sync will > be inaccurate and defeat the primary goal of this patch. If your > problem with clock_pelt is that the pending idle time is not accounted > for when entering idle but only at the next update (update blocked > load or wakeup of a thread). This patch below should fix this and > remove your A. That would help slightly the current situation, but this part is already covered by the estimator. > > > diff --git a/kernel/sched/pelt.h b/kernel/sched/pelt.h > index e06071bf3472..855877be4dd8 100644 > --- a/kernel/sched/pelt.h > +++ b/kernel/sched/pelt.h > @@ -114,6 +114,7 @@ static inline void update_idle_rq_clock_pelt(struct rq *rq) > { > u32 divider = ((LOAD_AVG_MAX - 1024) << SCHED_CAPACITY_SHIFT) > - LOAD_AVG_MAX; > u32 util_sum = rq->cfs.avg.util_sum; > + u64 now = rq_clock_task(rq); > util_sum += rq->avg_rt.util_sum; > util_sum += rq->avg_dl.util_sum; > > @@ -127,7 +128,10 @@ static inline void update_idle_rq_clock_pelt(struct rq *rq) > * rq's clock_task. > */ > if (util_sum >= divider) > - rq->lost_idle_time += rq_clock_task(rq) - rq->clock_pelt; > + rq->lost_idle_time += now - rq->clock_pelt; > + > + /* The rq is idle, we can sync to clock_task */ > + rq->clock_pelt = now; > } > > static inline u64 rq_clock_pelt(struct rq *rq) > > --- > > > > +} > > + > > /* > > * RQ-clock updating methods: > > */ > > @@ -674,6 +680,7 @@ static void update_rq_clock_task(struct rq *rq, s64 delta) > > update_irq_load_avg(rq, irq_delta + steal); > > #endif > > update_rq_clock_pelt(rq, delta); > > + update_rq_clock_pelt_lag(rq); > > } > > > > void update_rq_clock(struct rq *rq) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index 99ea9540ece4..046d5397eb8a 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -6852,6 +6852,14 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags) > > > > static void detach_entity_cfs_rq(struct sched_entity *se); > > > > +static u64 rq_clock_pelt_estimator(struct rq *rq, u64 last_update_time) > > +{ > > + u64 pelt_lag = sched_clock_cpu(cpu_of(rq)) - > > + u64_u32_load(rq->clock_pelt_lag); > > Have you evaluated the impact of calling sched_clock_cpu(cpu_of(rq)) > for a remote cpu ? especially with a huge number of migration and > concurrent access from several cpus I have not, but I will have a look. > > > + > > + return last_update_time + pelt_lag; > > +} > > + > > /* > > * Called immediately before a task is migrated to a new CPU; task_cpu(p) and > > * cfs_rq_of(p) references at time of call are still valid and identify the > > @@ -6859,6 +6867,9 @@ static void detach_entity_cfs_rq(struct sched_entity *se); > > */ > > static void migrate_task_rq_fair(struct task_struct *p, int new_cpu) > > { > > + struct sched_entity *se = &p->se; > > + struct rq *rq = task_rq(p); > > + > > /* > > * As blocked tasks retain absolute vruntime the migration needs to > > * deal with this by subtracting the old and adding the new > > @@ -6866,7 +6877,6 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu) > > * the task on the new runqueue. > > */ > > if (READ_ONCE(p->__state) == TASK_WAKING) { > > - struct sched_entity *se = &p->se; > > struct cfs_rq *cfs_rq = cfs_rq_of(se); > > > > se->vruntime -= u64_u32_load(cfs_rq->min_vruntime); > > @@ -6877,26 +6887,32 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu) > > * In case of TASK_ON_RQ_MIGRATING we in fact hold the 'old' > > * rq->lock and can modify state directly. > > */ > > - lockdep_assert_rq_held(task_rq(p)); > > - detach_entity_cfs_rq(&p->se); > > + lockdep_assert_rq_held(rq); > > + detach_entity_cfs_rq(se); > > > > } else { > > + u64 now; > > + > > + remove_entity_load_avg(se); > > + > > /* > > - * We are supposed to update the task to "current" time, then > > - * its up to date and ready to go to new CPU/cfs_rq. But we > > - * have difficulty in getting what current time is, so simply > > - * throw away the out-of-date time. This will result in the > > - * wakee task is less decayed, but giving the wakee more load > > - * sounds not bad. > > + * Here, the task's PELT values have been updated according to > > + * the current rq's clock. But if that clock hasn't been > > + * updated in a while, a substantial idle time will be missed, > > + * leading to an inflation after wake-up on the new rq. > > + * > > + * Estimate the PELT clock lag, and update sched_avg to ensure > > + * PELT continuity after migration. > > */ > > - remove_entity_load_avg(&p->se); > > + now = rq_clock_pelt_estimator(rq, se->avg.last_update_time); > > + __update_load_avg_blocked_se(now, se); > > } > > > > /* Tell new CPU we are migrated */ > > - p->se.avg.last_update_time = 0; > > + se->avg.last_update_time = 0; > > > > /* We have migrated, no longer consider this task hot */ > > - p->se.exec_start = 0; > > + se->exec_start = 0; > > > > update_scan_period(p, new_cpu); > > } > > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > > index f1a445efdc63..fdf2a9e54c0e 100644 > > --- a/kernel/sched/sched.h > > +++ b/kernel/sched/sched.h > > @@ -1027,8 +1027,13 @@ struct rq { > > /* Ensure that all clocks are in the same cache line */ > > u64 clock_task ____cacheline_aligned; > > u64 clock_pelt; > > + u64 clock_pelt_lag; > > unsigned long lost_idle_time; > > > > +#ifndef CONFIG_64BIT > > + u64 clock_pelt_lag_copy; > > +#endif > > + > > atomic_t nr_iowait; > > > > #ifdef CONFIG_SCHED_DEBUG > > -- > > 2.25.1 > >