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Date: Fri, 18 Dec 2020 14:12:50 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Xuewen Yan <xuewen.yan94@...il.com>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>,
Juri Lelli <juri.lelli@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
linux-kernel <linux-kernel@...r.kernel.org>,
Patrick Bellasi <patrick.bellasi@....com>,
Chunyan Zhang <zhang.lyra@...il.com>,
王科 (Ke Wang) <Ke.Wang@...soc.com>,
Ryan Y <xuewyan@...mail.com>,
Xuewen Yan <Xuewen.Yan@...soc.com>
Subject: Re: [PATCH v3] sched/fair: Avoid stale CPU util_est value for
schedutil in task dequeue
On Fri, 18 Dec 2020 at 10:28, Xuewen Yan <xuewen.yan94@...il.com> wrote:
>
> From: Xuewen Yan <xuewen.yan@...soc.com>
>
> CPU (root cfs_rq) estimated utilization (util_est) is currently used in
> dequeue_task_fair() to drive frequency selection before it is updated.
>
> with:
>
> CPU_util : rq->cfs.avg.util_avg
> CPU_util_est : rq->cfs.avg.util_est
> CPU_utilization : max(CPU_util, CPU_util_est)
> task_util : p->se.avg.util_avg
> task_util_est : p->se.avg.util_est
>
> dequeue_task_fair():
>
> /* (1) CPU_util and task_util update + inform schedutil about
> CPU_utilization changes */
> for_each_sched_entity() /* 2 loops */
> (dequeue_entity() ->) update_load_avg() -> cfs_rq_util_change()
> -> cpufreq_update_util() ->...-> sugov_update_[shared\|single]
> -> sugov_get_util() -> cpu_util_cfs()
>
> /* (2) CPU_util_est and task_util_est update */
> util_est_dequeue()
>
> cpu_util_cfs() uses CPU_utilization which could lead to a false (too
> high) utilization value for schedutil in task ramp-down or ramp-up
> scenarios during task dequeue.
>
> To mitigate the issue split the util_est update (2) into:
>
> (A) CPU_util_est update in util_est_dequeue()
> (B) task_util_est update in util_est_update()
>
> Place (A) before (1) and keep (B) where (2) is. The latter is necessary
> since (B) relies on task_util update in (1).
>
maybe add a
Fixes: 7f65ea42eb00 ("sched/fair: Add util_est on top of PELT")
> Signed-off-by: Xuewen Yan <xuewen.yan@...soc.com>
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>
Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
> Changes since v2:
> -modify the comment
> -move util_est_dequeue above within_margin()
> -modify the tab and space
>
> Changes since v1:
> -change the util_est_dequeue/update to inline type
> -use unsigned int enqueued rather than util_est in util_est_dequeue
> -remove "cpu" var
>
> ---
> kernel/sched/fair.c | 43 ++++++++++++++++++++++++++++---------------
> 1 file changed, 28 insertions(+), 15 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index ae7ceba..f3a1b7a 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3932,6 +3932,22 @@ static inline void util_est_enqueue(struct cfs_rq *cfs_rq,
> trace_sched_util_est_cfs_tp(cfs_rq);
> }
>
> +static inline void util_est_dequeue(struct cfs_rq *cfs_rq,
> + struct task_struct *p)
> +{
> + unsigned int enqueued;
> +
> + if (!sched_feat(UTIL_EST))
> + return;
> +
> + /* Update root cfs_rq's estimated utilization */
> + enqueued = cfs_rq->avg.util_est.enqueued;
> + enqueued -= min_t(unsigned int, enqueued, _task_util_est(p));
> + WRITE_ONCE(cfs_rq->avg.util_est.enqueued, enqueued);
> +
> + trace_sched_util_est_cfs_tp(cfs_rq);
> +}
> +
> /*
> * Check if a (signed) value is within a specified (unsigned) margin,
> * based on the observation that:
> @@ -3945,23 +3961,16 @@ static inline bool within_margin(int value, int margin)
> return ((unsigned int)(value + margin - 1) < (2 * margin - 1));
> }
>
> -static void
> -util_est_dequeue(struct cfs_rq *cfs_rq, struct task_struct *p, bool task_sleep)
> +static inline void util_est_update(struct cfs_rq *cfs_rq,
> + struct task_struct *p,
> + bool task_sleep)
> {
> long last_ewma_diff;
> struct util_est ue;
> - int cpu;
>
> if (!sched_feat(UTIL_EST))
> return;
>
> - /* Update root cfs_rq's estimated utilization */
> - ue.enqueued = cfs_rq->avg.util_est.enqueued;
> - ue.enqueued -= min_t(unsigned int, ue.enqueued, _task_util_est(p));
> - WRITE_ONCE(cfs_rq->avg.util_est.enqueued, ue.enqueued);
> -
> - trace_sched_util_est_cfs_tp(cfs_rq);
> -
> /*
> * Skip update of task's estimated utilization when the task has not
> * yet completed an activation, e.g. being migrated.
> @@ -4001,8 +4010,7 @@ static inline bool within_margin(int value, int margin)
> * To avoid overestimation of actual task utilization, skip updates if
> * we cannot grant there is idle time in this CPU.
> */
> - cpu = cpu_of(rq_of(cfs_rq));
> - if (task_util(p) > capacity_orig_of(cpu))
> + if (task_util(p) > capacity_orig_of(cpu_of(rq_of(cfs_rq))))
> return;
>
> /*
> @@ -4085,8 +4093,11 @@ static inline int newidle_balance(struct rq *rq, struct rq_flags *rf)
> util_est_enqueue(struct cfs_rq *cfs_rq, struct task_struct *p) {}
>
> static inline void
> -util_est_dequeue(struct cfs_rq *cfs_rq, struct task_struct *p,
> - bool task_sleep) {}
> +util_est_dequeue(struct cfs_rq *cfs_rq, struct task_struct *p) {}
> +
> +static inline void
> +util_est_update(struct cfs_rq *cfs_rq, struct task_struct *p,
> + bool task_sleep) {}
> static inline void update_misfit_status(struct task_struct *p, struct rq *rq) {}
>
> #endif /* CONFIG_SMP */
> @@ -5589,6 +5600,8 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> int idle_h_nr_running = task_has_idle_policy(p);
> bool was_sched_idle = sched_idle_rq(rq);
>
> + util_est_dequeue(&rq->cfs, p);
> +
> for_each_sched_entity(se) {
> cfs_rq = cfs_rq_of(se);
> dequeue_entity(cfs_rq, se, flags);
> @@ -5639,7 +5652,7 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> rq->next_balance = jiffies;
>
> dequeue_throttle:
> - util_est_dequeue(&rq->cfs, p, task_sleep);
> + util_est_update(&rq->cfs, p, task_sleep);
> hrtick_update(rq);
> }
>
> --
> 1.9.1
>
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