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SubjectRe: [PATCH 1/8] sched/fair: combine detach into dequeue when migrating task
On 2022/7/9 23:13, Chengming Zhou wrote:
> When we are migrating task out of the CPU, we can combine detach and
> propagation into dequeue_entity() to save the detach_entity_cfs_rq()
> in migrate_task_rq_fair().
>
> This optimization is like combining DO_ATTACH in the enqueue_entity()
> when migrating task to the CPU.
>
> So we don't have to traverse the CFS tree extra time to do the
> detach_entity_cfs_rq() -> propagate_entity_cfs_rq() call, which
> wouldn't be called anymore with this patch's change.
>
> detach_task()
> deactivate_task()
> dequeue_task_fair()
> for_each_sched_entity(se)
> dequeue_entity()
> update_load_avg() /* (1) */
> detach_entity_load_avg()
>
> set_task_cpu()
> migrate_task_rq_fair()
> detach_entity_cfs_rq() /* (2) */
> update_load_avg();
> detach_entity_load_avg();
> propagate_entity_cfs_rq();
> for_each_sched_entity()
> update_load_avg()
>
> This patch save the detach_entity_cfs_rq() called in (2) by doing
> the detach_entity_load_avg() for a CPU migrating task inside (1)
> (the task being the first se in the loop)
>
> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
> Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
> kernel/sched/fair.c | 30 +++++++++++++++++-------------
> 1 file changed, 17 insertions(+), 13 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a78d2e3b9d49..0689b94ed70b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4003,6 +4003,7 @@ static void detach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
> #define UPDATE_TG 0x1
> #define SKIP_AGE_LOAD 0x2
> #define DO_ATTACH 0x4
> +#define DO_DETACH 0x8
>
> /* Update task and its cfs_rq load average */
> static inline void update_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
> @@ -4020,7 +4021,14 @@ static inline void update_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
> decayed = update_cfs_rq_load_avg(now, cfs_rq);
> decayed |= propagate_entity_load_avg(se);
>
> - if (!se->avg.last_update_time && (flags & DO_ATTACH)) {
> + if (flags & DO_DETACH) {
> + /*
> + * DO_DETACH means we're here from dequeue_entity()
> + * and we are migrating task out of the CPU.
> + */
> + detach_entity_load_avg(cfs_rq, se);
> + update_tg_load_avg(cfs_rq);
> + } else if (!se->avg.last_update_time && (flags & DO_ATTACH)) {

Should I put "DO_DETACH" case after "DO_ATTACH" case? The "DO_ATTACH" maybe more likely, right?

Thanks.

>
> /*
> * DO_ATTACH means we're here from enqueue_entity().
> @@ -4292,6 +4300,7 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)
> #define UPDATE_TG 0x0
> #define SKIP_AGE_LOAD 0x0
> #define DO_ATTACH 0x0
> +#define DO_DETACH 0x0
>
> static inline void update_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se, int not_used1)
> {
> @@ -4511,6 +4520,11 @@ static __always_inline void return_cfs_rq_runtime(struct cfs_rq *cfs_rq);
> static void
> dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
> {
> + int action = UPDATE_TG;
> +
> + if (entity_is_task(se) && task_on_rq_migrating(task_of(se)))
> + action |= DO_DETACH;
> +
> /*
> * Update run-time statistics of the 'current'.
> */
> @@ -4524,7 +4538,7 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
> * - For group entity, update its weight to reflect the new share
> * of its group cfs_rq.
> */
> - update_load_avg(cfs_rq, se, UPDATE_TG);
> + update_load_avg(cfs_rq, se, action);
> se_update_runnable(se);
>
> update_stats_dequeue_fair(cfs_rq, se, flags);
> @@ -7076,8 +7090,6 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
> return new_cpu;
> }
>
> -static void detach_entity_cfs_rq(struct sched_entity *se);
> -
> /*
> * 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
> @@ -7099,15 +7111,7 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
> se->vruntime -= u64_u32_load(cfs_rq->min_vruntime);
> }
>
> - if (p->on_rq == TASK_ON_RQ_MIGRATING) {
> - /*
> - * 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(se);
> -
> - } else {
> + if (!task_on_rq_migrating(p)) {
> remove_entity_load_avg(se);
>
> /*

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