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Date: Wed, 29 Jun 2022 09:37:54 +0800
From: Chengming Zhou <zhouchengming@...edance.com>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: mingo@...hat.com, peterz@...radead.org, dietmar.eggemann@....com,
bsegall@...gle.com, vschneid@...hat.com,
linux-kernel@...r.kernel.org
Subject: Re: [External] Re: [PATCH v4] sched/fair: combine detach into dequeue
when migrating task
Hi, thanks for your review.
On 2022/6/28 23:26, Vincent Guittot wrote:
> On Mon, 20 Jun 2022 at 15:36, Chengming Zhou
> <zhouchengming@...edance.com> 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()
>> -> propagate_entity_cfs_rq() call in 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.
>> -> propagate_entity_cfs_rq() call in detach_entity_cfs_rq() in
>> migrate_task_rq_fair().
>>
>>
>> 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.
>>
>> Copied from Dietmar's much clearer comment:
>
> Although It's nice to keep the author of a comment, it should not be
> part of the commit message.
Ok, I will delete this.
>
>> 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()
>
> Could you add the full call stack so we can see more easily the extra
> loop that is saved
Good advice, I will add it.
Thanks.
>>
>> This patch save the propagate_entity_cfs_rq(&p->se) call from (2)
>> by doing the detach_entity_load_avg(), update_tg_load_avg() for
>> a migrating task inside (1) (the task being the first se in the loop)
>>
>> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
>
> Apart the small nit above:
>
> Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>
>
>> ---
>> v4:
>> - change the commit message a little.
>> - remove the forward declaration of detach_entity_cfs_rq()
>> - remove verbose comments in code.
>>
>> v3:
>> - change to use task_on_rq_migrating() and put Dietmar's much clearer
>> description in the commit message. Thanks!
>>
>> v2:
>> - fix !CONFIG_SMP build error
>> ---
>> 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 8bed75757e65..31d53c11e244 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -3931,6 +3931,7 @@ 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)
>> @@ -3948,7 +3949,14 @@ 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)) {
>>
>> /*
>> * DO_ATTACH means we're here from enqueue_entity().
>> @@ -4241,6 +4249,7 @@ 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)
>> {
>> @@ -4460,6 +4469,11 @@ 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'.
>> */
>> @@ -4473,7 +4487,7 @@ 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);
>> @@ -6938,8 +6952,6 @@ 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
>> @@ -6973,15 +6985,7 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
>> se->vruntime -= 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(&p->se);
>> -
>> - } else {
>> + if (!task_on_rq_migrating(p)) {
>> /*
>> * 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
>> --
>> 2.36.1
>>
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