Re: [External] Re: [PATCH v4] sched/fair: combine detach into dequeue when migrating task

[Chengming Zhou]

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@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> 
> Apart the small nit above:
> 
> Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> 
>> ---
>> 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
>>