Re: [PATCH v3] sched: fix wrong rq's runnable_avg update with rt task

[Paul Turner]

On Wed, Feb 13, 2013 at 12:54 AM, Vincent Guittot
<vincent.guittot@xxxxxxxxxx> wrote:
> When a RT task is scheduled on an idle CPU, the update of the rq's load is
> not done because CFS's functions are not called. Then, the idle_balance,
> which is called just before entering the idle function, updates the
> rq's load and makes the assumption that the elapsed time since the last
> update, was only running time.

So an interesting point here is we intrinsically treat RT time
differently in our discount of the CPU power.  It doesn't affect
anything in this patch per-say; but it should be kept in mind for
consistency when you do anything with the rq based numbers.

>
> The rq's load of a CPU that only runs a periodic RT task, is close to
> LOAD_AVG_MAX whatever the running duration of the RT task is.
>
> A new idle_exit function is called when the prev task is the idle function
> so the elapsed time will be accounted as idle time in the rq's load.
>
> Changes since V2:
> - remove useless definition for UP platform
> - rebased on top of Steven Rostedt's patches :
> https://lkml.org/lkml/2013/2/12/558
>
> Changes since V1:
> - move code out of schedule function and create a pre_schedule callback for
>   idle class instead.
>
> Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> ---
>  kernel/sched/fair.c      |   10 ++++++++++
>  kernel/sched/idle_task.c |    7 +++++++
>  kernel/sched/sched.h     |    1 +
>  3 files changed, 18 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 0fcdbff..6af5db3 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1562,6 +1562,16 @@ static inline void dequeue_entity_load_avg(struct cfs_rq *cfs_rq,
>                 se->avg.decay_count = atomic64_read(&cfs_rq->decay_counter);
>         } /* migrations, e.g. sleep=0 leave decay_count == 0 */
>  }
> +
> +/*
> + * Update the rq's load with the elapsed idle time before a task is
> + * scheduled. if the newly scheduled task is not a CFS task, idle_exit will
> + * be the only way to update the runnable statistic.
> + */
> +void idle_exit(int this_cpu, struct rq *this_rq)
> +{
> +       update_rq_runnable_avg(this_rq, 0);
> +}

This does creates a somewhat sneaky dependency CONFIG_FAIR_GROUP_SCHED
bits and sched-idle; and it kind of hides the problem by making the
subsequent update a nop.

Is the root of the problem perhaps that we assume we were previously
runnable in idle_balance in the first place?  Hmmm; this should
actually be essentially torn down by the dequeue of the last task in
the first place.  I'd have to think about it carefully but arguably we
might be able to just remove the update_rq_runnable_avg from
idle_balance entirely.

We'd still need to make sure we pushed updates against rq->avg to
cover RT task time (something you correctly point out is currently not
accounted, and which entry/exit from idle-task seems a reasonable
place); but this can then be independent of all the cgroup specific
bits.


>  #else
>  static inline void update_entity_load_avg(struct sched_entity *se,
>                                           int update_cfs_rq) {}
> diff --git a/kernel/sched/idle_task.c b/kernel/sched/idle_task.c
> index 66b5220..6e7e63c 100644
> --- a/kernel/sched/idle_task.c
> +++ b/kernel/sched/idle_task.c
> @@ -14,6 +14,12 @@ select_task_rq_idle(struct task_struct *p, int sd_flag, int flags)
>         return task_cpu(p); /* IDLE tasks as never migrated */
>  }
>
> +static void pre_schedule_idle(struct rq *rq, struct task_struct *prev)
> +{
> +       /* Update rq's load with elapsed idle time */
> +       idle_exit(smp_processor_id(), rq);
> +}
> +
>  static void post_schedule_idle(struct rq *rq)
>  {
>         idle_balance(smp_processor_id(), rq);
> @@ -95,6 +101,7 @@ const struct sched_class idle_sched_class = {
>
>  #ifdef CONFIG_SMP
>         .select_task_rq         = select_task_rq_idle,
> +       .pre_schedule           = pre_schedule_idle,
>         .post_schedule          = post_schedule_idle,
>  #endif
>
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index fc88644..5f26c93f 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -877,6 +877,7 @@ extern const struct sched_class idle_sched_class;
>
>  extern void trigger_load_balance(struct rq *rq, int cpu);
>  extern void idle_balance(int this_cpu, struct rq *this_rq);
> +extern void idle_exit(int this_cpu, struct rq *this_rq);
>
>  #else  /* CONFIG_SMP */
>
> --
> 1.7.9.5
>
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