Re: [PATCH] sched/fair: Rate limit calls to update_blocked_averages() for NOHZ

[Qais Yousef]

On 01/29/21 18:27, Vincent Guittot wrote:
> The patch below moves the update of the blocked load of CPUs outside newidle_balance().
> 
> Instead, the update is done with the usual idle load balance update. I'm working on an
> additonnal patch that will select this cpu that is about to become idle, instead of a
> random idle cpu but this 1st step fixe the problem of lot of update in newly idle.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 32 +++-----------------------------
>  1 file changed, 3 insertions(+), 29 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 197a51473e0c..8200b1d4df3d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7421,8 +7421,6 @@ enum migration_type {
>  #define LBF_NEED_BREAK	0x02
>  #define LBF_DST_PINNED  0x04
>  #define LBF_SOME_PINNED	0x08
> -#define LBF_NOHZ_STATS	0x10
> -#define LBF_NOHZ_AGAIN	0x20
>  
>  struct lb_env {
>  	struct sched_domain	*sd;
> @@ -8426,9 +8424,6 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>  	for_each_cpu_and(i, sched_group_span(group), env->cpus) {
>  		struct rq *rq = cpu_rq(i);
>  
> -		if ((env->flags & LBF_NOHZ_STATS) && update_nohz_stats(rq, false))
> -			env->flags |= LBF_NOHZ_AGAIN;
> -
>  		sgs->group_load += cpu_load(rq);
>  		sgs->group_util += cpu_util(i);
>  		sgs->group_runnable += cpu_runnable(rq);
> @@ -8969,11 +8964,6 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>  	struct sg_lb_stats tmp_sgs;
>  	int sg_status = 0;
>  
> -#ifdef CONFIG_NO_HZ_COMMON
> -	if (env->idle == CPU_NEWLY_IDLE && READ_ONCE(nohz.has_blocked))
> -		env->flags |= LBF_NOHZ_STATS;
> -#endif
> -
>  	do {
>  		struct sg_lb_stats *sgs = &tmp_sgs;
>  		int local_group;
> @@ -9010,15 +9000,6 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>  	/* Tag domain that child domain prefers tasks go to siblings first */
>  	sds->prefer_sibling = child && child->flags & SD_PREFER_SIBLING;
>  
> -#ifdef CONFIG_NO_HZ_COMMON
> -	if ((env->flags & LBF_NOHZ_AGAIN) &&
> -	    cpumask_subset(nohz.idle_cpus_mask, sched_domain_span(env->sd))) {
> -
> -		WRITE_ONCE(nohz.next_blocked,
> -			   jiffies + msecs_to_jiffies(LOAD_AVG_PERIOD));
> -	}
> -#endif
> -
>  	if (env->sd->flags & SD_NUMA)
>  		env->fbq_type = fbq_classify_group(&sds->busiest_stat);
>  
> @@ -10547,14 +10528,7 @@ static void nohz_newidle_balance(struct rq *this_rq)
>  		return;
>  
>  	raw_spin_unlock(&this_rq->lock);
> -	/*
> -	 * This CPU is going to be idle and blocked load of idle CPUs
> -	 * need to be updated. Run the ilb locally as it is a good
> -	 * candidate for ilb instead of waking up another idle CPU.
> -	 * Kick an normal ilb if we failed to do the update.
> -	 */
> -	if (!_nohz_idle_balance(this_rq, NOHZ_STATS_KICK, CPU_NEWLY_IDLE))

Since we removed the call to this function (which uses this_rq)

> -		kick_ilb(NOHZ_STATS_KICK);
> +	kick_ilb(NOHZ_STATS_KICK);

And unconditionally call kick_ilb() which will find a suitable cpu to run the
lb at regardless what this_rq is.

Doesn't the below become unnecessary now?

	  10494         /*
	  10495          * This CPU doesn't want to be disturbed by scheduler
	  10496          * housekeeping
	  10497          */
	  10498         if (!housekeeping_cpu(this_cpu, HK_FLAG_SCHED))
	  10499                 return;
	  10500
	  10501         /* Will wake up very soon. No time for doing anything else*/
	  10502         if (this_rq->avg_idle < sysctl_sched_migration_cost)
	  10503                 return;

And we can drop this_rq arg altogether?

>  	raw_spin_lock(&this_rq->lock);
>  }
>  
> @@ -10616,8 +10590,6 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>  			update_next_balance(sd, &next_balance);
>  		rcu_read_unlock();
>  
> -		nohz_newidle_balance(this_rq);
> -
>  		goto out;
>  	}
>  
> @@ -10683,6 +10655,8 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>  
>  	if (pulled_task)
>  		this_rq->idle_stamp = 0;
> +	else
> +		nohz_newidle_balance(this_rq);

Since nohz_newidle_balance() will not do any real work now, I couldn't figure
out what moving this here achieves. Fault from my end to parse the change most
likely :-)

Joel can still test this patch as is of course. This is just an early review
since I already spent the time trying to understand it.

Thanks

--
Qais Yousef

>  
>  	rq_repin_lock(this_rq, rf);
>  
> -- 
> 2.17.1