Re: [PATCH v3 1/3] sched: introduce a new migration flag to task_struct

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Oct 15, 2015 at 06:01:14PM +0900, byungchul.park@lge.com wrote:
> From: Byungchul Park <byungchul.park@lge.com>
> 
> This patch removes a weird coupling between se->avg.last_update_time and
> the condition checking for migration, and introduce a new migration flag.
> Now, scheduler can use the flag instead of se->avg.last_update_time to
> check if migration already happened or not.

Was there a problem with that coupling? This does not explain.

> Signed-off-by: Byungchul Park <byungchul.park@lge.com>
> ---
>  include/linux/sched.h |    3 +++
>  kernel/sched/core.c   |    1 +
>  kernel/sched/fair.c   |   22 ++++++++++++----------
>  kernel/sched/sched.h  |    1 +
>  4 files changed, 17 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 699228b..a104c72 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1379,6 +1379,9 @@ struct task_struct {
>  #endif
>  	int on_rq;
>  
> +	/* For indicating if a migration has happened. */
> +	int migrated;

You just created another 4 byte hole instead of filling one.

>  	int prio, static_prio, normal_prio;
>  	unsigned int rt_priority;
>  	const struct sched_class *sched_class;

> +++ b/kernel/sched/fair.c
> @@ -2771,14 +2771,15 @@ static void detach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
>  
>  /* Add the load generated by se into cfs_rq's load average */
>  static inline void
> -enqueue_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
> +enqueue_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
>  {
>  	struct sched_avg *sa = &se->avg;
>  	u64 now = cfs_rq_clock_task(cfs_rq);
> -	int migrated, decayed;
> +	int decayed;
> +	int migrated = flags & ENQUEUE_MIGRATED;
> +	int created = !sa->last_update_time;
>  
> -	migrated = !sa->last_update_time;
> -	if (!migrated) {
> +	if (!migrated && !created) {
>  		__update_load_avg(now, cpu_of(rq_of(cfs_rq)), sa,
>  			se->on_rq * scale_load_down(se->load.weight),
>  			cfs_rq->curr == se, NULL);
> @@ -2789,10 +2790,10 @@ enqueue_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  	cfs_rq->runnable_load_avg += sa->load_avg;
>  	cfs_rq->runnable_load_sum += sa->load_sum;
>  
> -	if (migrated)
> +	if (migrated || created)
>  		attach_entity_load_avg(cfs_rq, se);
>  
> -	if (decayed || migrated)
> +	if (decayed || migrated || created)
>  		update_tg_load_avg(cfs_rq, 0);
>  }

How much extra code gets generated for this? These _are_ hot paths.

> @@ -4136,6 +4137,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>  	struct cfs_rq *cfs_rq;
>  	struct sched_entity *se = &p->se;
>  
> +	flags = flags | (xchg(&p->migrated, 0) ? ENQUEUE_MIGRATED : 0);

Yeah, no way. xchg() is an absurdly expensive instruction, we do not
place that unconditionally in the enqueue path.

> @@ -5021,7 +5023,7 @@ static void migrate_task_rq_fair(struct task_struct *p, int next_cpu)
>  	remove_entity_load_avg(&p->se);
>  
>  	/* Tell new CPU we are migrated */
> -	p->se.avg.last_update_time = 0;
> +	p->migrated = 1;
>  
>  	/* We have migrated, no longer consider this task hot */
>  	p->se.exec_start = 0;
> @@ -8082,7 +8084,7 @@ static void task_move_group_fair(struct task_struct *p)
>  	set_task_rq(p, task_cpu(p));
>  
>  #ifdef CONFIG_SMP
> -	/* Tell se's cfs_rq has been changed -- migrated */
> +	/* Tell se's cfs_rq has been changed */
>  	p->se.avg.last_update_time = 0;
>  #endif
>  	attach_task_cfs_rq(p);

So my tiny little patch removed more code than it added, and simplified
a few things, like the above. Now we have 2 states to worry about.

How is this making things better?

> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index af6f252..66d0552 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1158,6 +1158,7 @@ static const u32 prio_to_wmult[40] = {
>  #define ENQUEUE_WAKING		0
>  #endif
>  #define ENQUEUE_REPLENISH	8
> +#define ENQUEUE_MIGRATED	16

Won't actually apply that..