linux-kernel - Re: [PATCH v2 1/3] sched: Stop nohz stats when decayed

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Date:   Thu, 8 Feb 2018 14:15:22 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>, mingo@...nel.org,
        linux-kernel@...r.kernel.org, morten.rasmussen@...s.arm.com,
        brendan.jackman@....com, dietmar.eggemann@....com
Subject: Re: [PATCH v2 1/3] sched: Stop nohz stats when decayed

On Thu, Feb 08, 2018 at 12:46:53PM +0000, Valentin Schneider wrote:
> On 02/06/2018 07:23 PM, Vincent Guittot wrote:
> > [...]
> > @@ -7826,8 +7842,8 @@ 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);
> > +		if ((env->flags & LBF_NOHZ_STATS) && update_nohz_stats(rq))
> > +			env->flags |= LBF_NOHZ_AGAIN;
> >  
> >  		/* Bias balancing toward cpus of our domain */
> >  		if (local_group)
> > @@ -7979,18 +7995,15 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
> >  	struct sg_lb_stats *local = &sds->local_stat;
> >  	struct sg_lb_stats tmp_sgs;
> >  	int load_idx, prefer_sibling = 0;
> > +	int has_blocked = READ_ONCE(nohz.has_blocked);
> >  	bool overload = false;
> >  
> >  	if (child && child->flags & SD_PREFER_SIBLING)
> >  		prefer_sibling = 1;
> >  
> >  #ifdef CONFIG_NO_HZ_COMMON
> > -	if (env->idle == CPU_NEWLY_IDLE) {
> > +	if (env->idle == CPU_NEWLY_IDLE && has_blocked)
> >  		env->flags |= LBF_NOHZ_STATS;
> > -
> > -		if (cpumask_subset(nohz.idle_cpus_mask, sched_domain_span(env->sd)))
> > -			nohz.next_stats = jiffies + msecs_to_jiffies(LOAD_AVG_PERIOD);
> > -	}
> >  #endif
> >  
> >  	load_idx = get_sd_load_idx(env->sd, env->idle);
> > @@ -8046,6 +8059,15 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
> >  		sg = sg->next;
> >  	} while (sg != env->sd->groups);
> >  
> > +#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));
> 
> Here we push the stats update forward if we visited all the nohz CPUs but they
> still have blocked load. IMO we should also clear the nohz.has_blocked flag
> if we visited all the nohz CPUs and none had blocked load left.

You can't, new cpu's can have joined the set. I used to detect that, but
that requires atomic ops.