Commit 8c1f560c authored by Xuewen Yan's avatar Xuewen Yan Committed by Peter Zijlstra
Browse files

sched/fair: Avoid stale CPU util_est value for schedutil in task dequeue 



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).

Fixes: 7f65ea42 ("sched/fair: Add util_est on top of PELT")
Signed-off-by: default avatarXuewen Yan <xuewen.yan@unisoc.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarDietmar Eggemann <dietmar.eggemann@arm.com>
Reviewed-by: default avatarVincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/1608283672-18240-1-git-send-email-xuewen.yan94@gmail.com
parent 0301925d

kernel/sched/fair.c

+28 −15
Original line number Diff line number Diff line
@@ -3943,6 +3943,22 @@ 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:
@@ -3956,23 +3972,16 @@ 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.
@@ -4012,8 +4021,7 @@ util_est_dequeue(struct cfs_rq *cfs_rq, struct task_struct *p, bool task_sleep) 
	 * 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;



	/*
@@ -4096,7 +4104,10 @@ static inline void 
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,

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) {}
@@ -5609,6 +5620,8 @@ 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);
@@ -5659,7 +5672,7 @@ 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);

}