Re: [PATCH 4/4] sched,fair: Fix PELT integrity for new tasks

[Vincent Guittot <vincent.guittot@linaro.org>]

On 21 June 2016 at 15:17, Peter Zijlstra <peterz@infradead.org> wrote:
> On Tue, Jun 21, 2016 at 10:41:19AM +0200, Peter Zijlstra wrote:
>> On Mon, Jun 20, 2016 at 03:49:34PM +0100, Dietmar Eggemann wrote:
>> > On 20/06/16 13:35, Vincent Guittot wrote:
>>
>> > > It will go through wake_up_new_task and post_init_entity_util_avg
>> > > during its fork which is enough to set last_update_time. Then, it will
>> > > use the switched_to_fair if the task becomes a fair one
>> >
>> > Oh I see. We want to make sure that every task (even when forked as
>> > !fair) has a last_update_time value != 0, when becoming fair one day.
>>
>> Right, see 2 below. I need to write a bunch of comments explaining PELT
>> proper, as well as document these things.
>>
>> The things we ran into with these patches were that:
>>
>>  1) You need to update the cfs_rq _before_ any entity attach/detach
>>     (and might need to update_tg_load_avg when update_cfs_rq_load_avg()
>>     returns true).
>>
>>  2) (fair) entities are always attached, switched_from/to deal with !fair.
>>
>>  3) cpu migration is the only exception and uses the last_update_time=0
>>     thing -- because refusal to take second rq->lock.
>>
>> Which is why I dislike Yuyang's patches, they create more exceptions
>> instead of applying existing rules (albeit undocumented).
>>
>> Esp. 1 is important, because while for mathematically consistency you
>> don't actually need to do this, you only need the entities to be
>> up-to-date with the cfs rq when you attach/detach, but that forgets the
>> temporal aspect of _when_ you do this.
>
> I have the below for now, I'll continue poking at this for a bit.
>
>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -692,6 +692,7 @@ void init_entity_runnable_average(struct
>
>  static inline u64 cfs_rq_clock_task(struct cfs_rq *cfs_rq);
>  static int update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq, bool update_freq);
> +static void update_tg_load_avg(struct cfs_rq *cfs_rq, int force);
>  static void attach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se);
>
>  /*
> @@ -757,7 +758,8 @@ void post_init_entity_util_avg(struct sc
>                 }
>         }
>
> -       update_cfs_rq_load_avg(now, cfs_rq, false);
> +       if (update_cfs_rq_load_avg(now, cfs_rq, false))
> +               update_tg_load_avg(cfs_rq, false);

You should move update_tg_load_avg after attach_entity_load_avg to
take into account the newly attached task

>         attach_entity_load_avg(cfs_rq, se);
>  }
>
> @@ -2919,7 +2921,21 @@ static inline void cfs_rq_util_change(st
>         WRITE_ONCE(*ptr, res);                                  \
>  } while (0)
>
> -/* Group cfs_rq's load_avg is used for task_h_load and update_cfs_share */
> +/**
> + * update_cfs_rq_load_avg - update the cfs_rq's load/util averages
> + * @now: current time, as per cfs_rq_clock_task()
> + * @cfs_rq: cfs_rq to update
> + * @update_freq: should we call cfs_rq_util_change() or will the call do so
> + *
> + * The cfs_rq avg is the direct sum of all its entities (blocked and runnable)
> + * avg. The immediate corollary is that all (fair) tasks must be attached, see
> + * post_init_entity_util_avg().
> + *
> + * cfs_rq->avg is used for task_h_load() and update_cfs_share() for example.
> + *
> + * Returns true if the load decayed or we removed utilization. It is expected
> + * that one calls update_tg_load_avg() on this condition.
> + */
>  static inline int
>  update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq, bool update_freq)
>  {
> @@ -2974,6 +2990,14 @@ static inline void update_load_avg(struc
>                 update_tg_load_avg(cfs_rq, 0);
>  }
>
> +/**
> + * attach_entity_load_avg - attach this entity to its cfs_rq load avg
> + * @cfs_rq: cfs_rq to attach to
> + * @se: sched_entity to attach
> + *
> + * Must call update_cfs_rq_load_avg() before this, since we rely on
> + * cfs_rq->avg.last_update_time being current.
> + */
>  static void attach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  {
>         if (!sched_feat(ATTACH_AGE_LOAD))
> @@ -3005,6 +3029,14 @@ static void attach_entity_load_avg(struc
>         cfs_rq_util_change(cfs_rq);
>  }
>
> +/**
> + * detach_entity_load_avg - detach this entity from its cfs_rq load avg
> + * @cfs_rq: cfs_rq to detach from
> + * @se: sched_entity to detach
> + *
> + * Must call update_cfs_rq_load_avg() before this, since we rely on
> + * cfs_rq->avg.last_update_time being current.
> + */
>  static void detach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  {
>         __update_load_avg(cfs_rq->avg.last_update_time, cpu_of(rq_of(cfs_rq)),
> @@ -8392,7 +8424,8 @@ static void detach_task_cfs_rq(struct ta
>         }
>
>         /* Catch up with the cfs_rq and remove our load when we leave */
> -       update_cfs_rq_load_avg(now, cfs_rq, false);
> +       if (update_cfs_rq_load_avg(now, cfs_rq, false))
> +               update_tg_load_avg(cfs_rq, false);

same as post_init_entity_util_avg. You should put it after the
detach_entity_load_avg

>         detach_entity_load_avg(cfs_rq, se);
>  }
>
> @@ -8411,7 +8444,8 @@ static void attach_task_cfs_rq(struct ta
>  #endif
>
>         /* Synchronize task with its cfs_rq */
> -       update_cfs_rq_load_avg(now, cfs_rq, false);
> +       if (update_cfs_rq_load_avg(now, cfs_rq, false))
> +               update_tg_load_avg(cfs_rq, false);

same as post_init_entity_util_avg

>         attach_entity_load_avg(cfs_rq, se);
>
>         if (!vruntime_normalized(p))