Re: [PATCH RFC] sched/fair: let cpu's cfs_rq to reflect task migration

[Dietmar Eggemann]

On 06/04/16 09:37, Morten Rasmussen wrote:
> On Tue, Apr 05, 2016 at 06:00:40PM +0100, Dietmar Eggemann wrote:
>> @@ -2893,8 +2906,12 @@ static void attach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
>>         se->avg.last_update_time = cfs_rq->avg.last_update_time;
>>         cfs_rq->avg.load_avg += se->avg.load_avg;
>>         cfs_rq->avg.load_sum += se->avg.load_sum;
>> -       cfs_rq->avg.util_avg += se->avg.util_avg;
>> -       cfs_rq->avg.util_sum += se->avg.util_sum;
>> +
>> +       if (!entity_is_task(se))
>> +               return;
>> +
>> +       rq_of(cfs_rq)->cfs.avg.util_avg += se->avg.util_avg;
>> +       rq_of(cfs_rq)->cfs.avg.util_sum += se->avg.util_sum;
> 
> To me it seems that you cannot be sure that the rq_of(cfs_rq)->cfs.avg
> time stamp is aligned with se->avg time stamp, which is necessary before
> you can add/subtract two geometric series without introducing an error.
> 
> attach_entity_load_avg() is called (through a couple of other functions)
> from the for_each_sched_entity() loop in enqueue_task_fair() which works
> its way towards the root cfs_rq, i.e. rq_of(cfs_rq)->cfs. So in the loop
> iteration where you attach the task sched_entity, we haven't yet visited
> and updated rq_of(cfs_rq)->cfs.avg.
> 
> If you just add the task contribution and discover later that there is a
> time delta when you update rq_of(cfs_rq)->cfs.avg you end up decaying
> the task contribution which was already up-to-date and its util
> contribution to rq_of(cfs_rq)->cfs.avg ends up being smaller than it
> should be.
> 
> Am I missing something?

No that's definitely wrong in the current patch. I could defer the attach into
the update_cfs_rq_load_avg() call for the root cfs_rq if 
'&rq_of(cfs_rq)->cfs != cfs_rq' in attach_entity_load_avg():

Something like this (only lightly tested!):

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 51d675715776..d31d9cd453a1 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2856,6 +2856,16 @@ static inline int update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq)
        decayed = __update_load_avg(now, cpu_of(rq_of(cfs_rq)), sa,
                scale_load_down(cfs_rq->load.weight), cfs_rq->curr != NULL, cfs_rq);
 
+       if (cfs_rq->added_util_avg) {
+               sa->util_avg += cfs_rq->added_util_avg;
+               cfs_rq->added_util_avg = 0;
+       }
+
+       if (cfs_rq->added_util_sum) {
+               sa->util_sum += cfs_rq->added_util_sum;
+               cfs_rq->added_util_sum = 0;
+       }
+
 #ifndef CONFIG_64BIT
        smp_wmb();
        cfs_rq->load_last_update_time_copy = sa->last_update_time;
@@ -2910,8 +2920,13 @@ static void attach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
        if (!entity_is_task(se))
                return;
 
-       rq_of(cfs_rq)->cfs.avg.util_avg += se->avg.util_avg;
-       rq_of(cfs_rq)->cfs.avg.util_sum += se->avg.util_sum;
+       if (&rq_of(cfs_rq)->cfs == cfs_rq) {
+               rq_of(cfs_rq)->cfs.avg.util_avg += se->avg.util_avg;
+               rq_of(cfs_rq)->cfs.avg.util_sum += se->avg.util_sum;
+       } else {
+               rq_of(cfs_rq)->cfs.added_util_avg = se->avg.util_avg;
+               rq_of(cfs_rq)->cfs.added_util_sum = se->avg.util_sum;
+       }
 }
 
 static void detach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
@@ -4344,8 +4359,11 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
                cfs_rq = cfs_rq_of(se);
                cfs_rq->h_nr_running++;
 
-               if (cfs_rq_throttled(cfs_rq))
+               if (cfs_rq_throttled(cfs_rq)) {
+                       rq_of(cfs_rq)->cfs.added_util_avg = 0;
+                       rq_of(cfs_rq)->cfs.added_util_sum = 0;
                        break;
+               }

                update_load_avg(se, 1);
                update_cfs_shares(cfs_rq);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index b2ff5a2bd6df..f0ea3a7eaf07 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -391,6 +391,8 @@ struct cfs_rq {
        unsigned long tg_load_avg_contrib;
 #endif
        atomic_long_t removed_load_avg, removed_util_avg;
+       u32 added_util_sum;
+       unsigned long added_util_avg;
 #ifndef CONFIG_64BIT
        u64 load_last_update_time_copy;

But attach_entity_load_avg() is not only called in enqueue_entity_load_avg() for migrated
tasks but also in attach_task_cfs_rq() which is called from switched_to_fair() and
task_move_group_fair() where we can't assume that after the enqueue_entity_load_avg() a
call to update_cfs_rq_load_avg() follows like in

enqueue_task_fair():

    for_each_sched_entity(se)
        enqueue_entity()
	    enqueue_entity_load_avg()
        	    update_cfs_rq_load_avg(now, cfs_rq)
                    if (migrated) attach_entity_load_avg()     

    for_each_sched_entity(se)
        update_load_avg()
            update_cfs_rq_load_avg(now, cfs_rq)

	  
Not sure if we can just update the root cfs_rq to se->avg.last_update_time before we add
se->avg.util_[avg/sum] to rq_of(cfs_rq)->cfs.avg.util_[avg/sum] in attach_entity_load_avg()?

cfs_rq throttling has to be considered as well ...