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Subject: Re: [PATCH 1/3] sched: Stop nohz stats when decayed
To:     Vincent Guittot <vincent.guittot@linaro.org>, peterz@infradead.org,
        mingo@kernel.org, linux-kernel@vger.kernel.org
Cc:     morten.rasmussen@foss.arm.com, brendan.jackman@arm.com,
        dietmar.eggemann@arm.com
References: <20180201181041.GF2269@hirez.programming.kicks-ass.net>
 <1517905943-24528-1-git-send-email-vincent.guittot@linaro.org>
From:   Valentin Schneider <valentin.schneider@arm.com>
Message-ID: <352e52af-1f2e-9b87-b94f-c5a4313d8b08@arm.com>
Date:   Tue, 6 Feb 2018 14:16:24 +0000
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Hi Vincent,

On 02/06/2018 08:32 AM, Vincent Guittot wrote:
> Stopped the periodic update of blocked load when all idle CPUs have fully
> decayed. We introduce a new nohz.has_blocked that reflect if some idle
> CPUs has blocked load that have to be periodiccally updated. nohz.has_blocked
> is set everytime that a Idle CPU can have blocked load and it is then clear
> when no more blocked load has been detected during an update. We don't need
> atomic operation but only to make cure of the right ordering when updating
> nohz.idle_cpus_mask and nohz.has_blocked.
> 
> Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
>  kernel/sched/fair.c  | 94 +++++++++++++++++++++++++++++++++++++++++-----------
>  kernel/sched/sched.h |  1 +
>  2 files changed, 75 insertions(+), 20 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 7af1fa9..279f4b2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5383,8 +5383,9 @@ decay_load_missed(unsigned long load, unsigned long missed_updates, int idx)
>  static struct {
>  	cpumask_var_t idle_cpus_mask;
>  	atomic_t nr_cpus;
> +	int has_blocked;		/* Idle CPUS has blocked load */
>  	unsigned long next_balance;     /* in jiffy units */
> -	unsigned long next_stats;
> +	unsigned long next_blocked;	/* Next update of blocked load in jiffies */
>  } nohz ____cacheline_aligned;
>  
>  #endif /* CONFIG_NO_HZ_COMMON */
> @@ -6951,6 +6952,7 @@ enum fbq_type { regular, remote, all };
>  #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;
> @@ -7335,8 +7337,6 @@ static void attach_tasks(struct lb_env *env)
>  	rq_unlock(env->dst_rq, &rf);
>  }
>  
> -#ifdef CONFIG_FAIR_GROUP_SCHED
> -
>  static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)
>  {
>  	if (cfs_rq->load.weight)
> @@ -7354,11 +7354,14 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)
>  	return true;
>  }
>  
> +#ifdef CONFIG_FAIR_GROUP_SCHED
> +
>  static void update_blocked_averages(int cpu)
>  {
>  	struct rq *rq = cpu_rq(cpu);
>  	struct cfs_rq *cfs_rq, *pos;
>  	struct rq_flags rf;
> +	bool done = true;
>  
>  	rq_lock_irqsave(rq, &rf);
>  	update_rq_clock(rq);
> @@ -7388,10 +7391,14 @@ static void update_blocked_averages(int cpu)
>  		 */
>  		if (cfs_rq_is_decayed(cfs_rq))
>  			list_del_leaf_cfs_rq(cfs_rq);
> +		else
> +			done = false;
>  	}
>  
>  #ifdef CONFIG_NO_HZ_COMMON
>  	rq->last_blocked_load_update_tick = jiffies;
> +	if (done)
> +		rq->has_blocked_load = 0;
>  #endif
>  	rq_unlock_irqrestore(rq, &rf);
>  }
> @@ -7454,6 +7461,8 @@ static inline void update_blocked_averages(int cpu)
>  	update_cfs_rq_load_avg(cfs_rq_clock_task(cfs_rq), cfs_rq);
>  #ifdef CONFIG_NO_HZ_COMMON
>  	rq->last_blocked_load_update_tick = jiffies;
> +	if (cfs_rq_is_decayed(cfs_rq))
> +		rq->has_blocked_load = 0;
>  #endif
>  	rq_unlock_irqrestore(rq, &rf);
>  }
> @@ -7789,18 +7798,25 @@ group_type group_classify(struct sched_group *group,
>  	return group_other;
>  }
>  
> -static void update_nohz_stats(struct rq *rq)
> +static bool update_nohz_stats(struct rq *rq)
>  {
>  #ifdef CONFIG_NO_HZ_COMMON
>  	unsigned int cpu = rq->cpu;
>  
> +	if (!rq->has_blocked_load)
> +		return false;
> +
>  	if (!cpumask_test_cpu(cpu, nohz.idle_cpus_mask))
> -		return;
> +		return false;
>  
>  	if (!time_after(jiffies, rq->last_blocked_load_update_tick))

I forgot to ask this on the initial thread - what's the point of this condition ? At first glance I would have said that this would make more sense:

if (!time_after(jiffies, rq->last_blocked_load_update_tick + msecs_to_jiffies(LOAD_AVG_PERIOD))
	return false;

But maybe it's simply there to skip an update if it has already been done in the same jiffy interval ?

> -		return;
> +		return true;
>  
>  	update_blocked_averages(cpu);
> +
> +	return rq->has_blocked_load;
> +#else
> +	return false;
>  #endif
>  }
>  
> @@ -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));
> +	}
> +#endif
> +
>  	if (env->sd->flags & SD_NUMA)
>  		env->fbq_type = fbq_classify_group(&sds->busiest_stat);
>  
> @@ -9069,6 +9091,8 @@ static void nohz_balancer_kick(struct rq *rq)
>  	struct sched_domain *sd;
>  	int nr_busy, i, cpu = rq->cpu;
>  	unsigned int flags = 0;
> +	unsigned long has_blocked = READ_ONCE(nohz.has_blocked);
> +	unsigned long next = READ_ONCE(nohz.next_blocked);

What about something slightly more explicit, e.g. next_stats/next_blocked ? There's also nohz.next_balance referenced here so IMO it's best to keep things clear.

>  
>  	if (unlikely(rq->idle_balance))
>  		return;
> @@ -9086,7 +9110,7 @@ static void nohz_balancer_kick(struct rq *rq)
>  	if (likely(!atomic_read(&nohz.nr_cpus)))
>  		return;
>  
> -	if (time_after(now, nohz.next_stats))
> +	if (time_after(now, next) && has_blocked)
>  		flags = NOHZ_STATS_KICK;
>  
>  	if (time_before(now, nohz.next_balance))
> @@ -9207,13 +9231,15 @@ void nohz_balance_enter_idle(int cpu)
>  	if (!housekeeping_cpu(cpu, HK_FLAG_SCHED))
>  		return;
>  
> +	rq->has_blocked_load = 1;
> +
>  	if (rq->nohz_tick_stopped)
> -		return;
> +		goto out;
>  
>  	/*
>  	 * If we're a completely isolated CPU, we don't play.
>  	 */
> -	if (on_null_domain(cpu_rq(cpu)))
> +	if (on_null_domain(rq))
>  		return;
>  
>  	rq->nohz_tick_stopped = 1;
> @@ -9222,6 +9248,13 @@ void nohz_balance_enter_idle(int cpu)
>  	atomic_inc(&nohz.nr_cpus);
>  
>  	set_cpu_sd_state_idle(cpu);
> +
> +out:
> +	/*
> +	 * Each time a cpu enter idle, we assume that it has blocked load and
> +	 * enable the periodic update of the load of idle cpus
> +	 */
> +	WRITE_ONCE(nohz.has_blocked, 1);
>  }
>  #else
>  static inline void nohz_balancer_kick(struct rq *rq) { }
> @@ -9374,6 +9407,16 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>  
>  	SCHED_WARN_ON((flags & NOHZ_KICK_MASK) == NOHZ_BALANCE_KICK);
>  
> +	/*
> +	 * We assume there will be no idle load after this update and clear
> +	 * the stats state. If a cpu enters idle in the mean time, it will

s/stats state/has_blocked flag/ (repeated a few times in the comment block)

> +	 * set the stats state and trig another update of idle load.
> +	 * Because a cpu that becomes idle, is added to idle_cpus_mask before
> +	 * setting the stats state, we are sure to not clear the state and not
> +	 * check the load of an idle cpu.
> +	 */
> +	WRITE_ONCE(nohz.has_blocked, 0);
> +
>  	for_each_cpu(balance_cpu, nohz.idle_cpus_mask) {
>  		if (balance_cpu == this_cpu || !idle_cpu(balance_cpu))
>  			continue;
> @@ -9383,11 +9426,16 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>  		 * work being done for other cpus. Next load
>  		 * balancing owner will pick it up.
>  		 */
> -		if (need_resched())
> -			break;
> +		if (need_resched()) {
> +			has_blocked_load = true;
> +			goto abort;
> +		}
>  
>  		rq = cpu_rq(balance_cpu);
>  
> +		update_blocked_averages(rq->cpu);
> +		has_blocked_load |= rq->has_blocked_load;
> +
>  		/*
>  		 * If time for next balance is due,
>  		 * do the balance.
> @@ -9400,7 +9448,6 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>  			cpu_load_update_idle(rq);
>  			rq_unlock_irq(rq, &rf);
>  
> -			update_blocked_averages(rq->cpu);
>  			if (flags & NOHZ_BALANCE_KICK)
>  				rebalance_domains(rq, CPU_IDLE);
>  		}
> @@ -9415,7 +9462,13 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>  	if (flags & NOHZ_BALANCE_KICK)
>  		rebalance_domains(this_rq, CPU_IDLE);
>  
> -	nohz.next_stats = next_stats;
> +	WRITE_ONCE(nohz.next_blocked,
> +		now + msecs_to_jiffies(LOAD_AVG_PERIOD));
> +
> +abort:
> +	/* There is still blocked load, enable periodic update */
> +	if (has_blocked_load)
> +		WRITE_ONCE(nohz.has_blocked, 1);
>  
>  	/*
>  	 * next_balance will be updated only when there is a need.
> @@ -10046,6 +10099,7 @@ __init void init_sched_fair_class(void)
>  
>  #ifdef CONFIG_NO_HZ_COMMON
>  	nohz.next_balance = jiffies;
> +	nohz.next_blocked = jiffies;
>  	zalloc_cpumask_var(&nohz.idle_cpus_mask, GFP_NOWAIT);
>  #endif
>  #endif /* SMP */
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index e200045..ad9b929 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -723,6 +723,7 @@ struct rq {
>  #ifdef CONFIG_SMP
>  	unsigned long last_load_update_tick;
>  	unsigned long last_blocked_load_update_tick;
> +	unsigned int has_blocked_load;
>  #endif /* CONFIG_SMP */
>  	unsigned int nohz_tick_stopped;
>  	atomic_t nohz_flags;
>