Re: [PATCH v2 2/2] sched/fair: Update blocked load from newly idle balance

[Dietmar Eggemann]

On 12/20/2017 03:22 PM, Peter Zijlstra wrote:
> On Fri, Dec 01, 2017 at 06:01:57PM +0000, Brendan Jackman wrote:
> > @@ -7913,6 +7928,29 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
> >   	if (child && child->flags & SD_PREFER_SIBLING)
> >   		prefer_sibling = 1;
> >   
> > +#ifdef CONFIG_NO_HZ_COMMON
> > +	if (env->idle == CPU_NEWLY_IDLE) {
> > +		int cpu;
> > +
> > +		/* Update the stats of NOHZ idle CPUs in the sd */
> > +		for_each_cpu_and(cpu, sched_domain_span(env->sd),
> > +				 nohz.idle_cpus_mask) {
> > +			struct rq *rq = cpu_rq(cpu);
> > +
> > +			/* ... Unless we've already done since the last tick */
> > +			if (time_after(jiffies, rq->last_blocked_load_update_tick))
> > +				update_blocked_averages(cpu);
> > +		}
> > +	}
> > +	/*
> > +	 * If we've just updated all of the NOHZ idle CPUs, then we can push
> > +	 * back the next nohz.next_update, which will prevent an unnecessary
> > +	 * wakeup for the nohz stats kick
> > +	 */
> > +	if (cpumask_subset(nohz.idle_cpus_mask, sched_domain_span(env->sd)))
> > +		nohz.next_update = jiffies + LOAD_AVG_PERIOD;
> > +#endif
> > +
> >   	load_idx = get_sd_load_idx(env->sd, env->idle);
> >   
> >   	do {
>
> We're already going to be iterating all those CPUs through
> update_sg_lb_stats(), why not push it all the way down there and avoid
> the double iteration?

Brendan doesn't work for ARM anymore although he might still follow this 
discussion.

So you think that we can do the time_after(jiffies, 
rq->last_blocked_load_update_tick) check in update_sg_lb_stats() and 
possibly the update_blocked_averages() for all the cpus in the sd and 
then only update nohz.next_update in update_sd_lb_stats(). Let me try 
this ...