Date:   Mon, 22 Jan 2018 09:40:41 +0000
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Brendan Jackman <brendan.jackman@....com>,
        Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [RFC PATCH 2/5] sched: Add NOHZ_STATS_KICK

On 01/15/2018 08:26 AM, Vincent Guittot wrote:
> Le Wednesday 03 Jan 2018 à 10:16:00 (+0100), Vincent Guittot a écrit :
>> Hi Peter,
>>
>> On 22 December 2017 at 21:42, Peter Zijlstra <peterz@...radead.org> wrote:
>>> On Fri, Dec 22, 2017 at 07:56:29PM +0100, Peter Zijlstra wrote:
>>>> Right; but I figured we'd try and do it 'right' and see how horrible it
>>>> is before we try and do funny things.
>>>
>>> So now it should have a 32ms tick for up to .5s when the system goes
>>> completely idle.
>>>
>>> No idea how bad that is..
>>
>> I have tested your branch but the timer doesn't seem to fire correctly
>> because i can still see blocked load in the use case i have run.
>> I haven't found the reason yet

Isn't the issue with the timer based implementation that 
rq->has_blocked_load is never set to 1 ?

Something you changed in your implementation by adding a 
rq->has_blocked_load = 1 into nohz_balance_enter_idle().

> 
> Hi Peter,
> 
> With the patch below on top of your branch, the blocked loads are updated and
> decayed regularly. The main differences are:
> - It doesn't use a timer to trig ilb but the tick and when a cpu becomes idle.
>    The main drawback of this solution is that the load is blocked when the
>    system is fully idle with the advantage of not waking up a fully idle
>    system. We have to wait for the next tick or newly idle event for updating
>    blocked load when the system leaves idle stat which can be up to a tick long.
>    If this is too long, we can check for kicking ilb when task wakes up so the
>    blocked load will be updated as soon as the system leaves idle state.
>    The main advantage is that we don't wake up a fully idle system every 32ms to
>    update blocked load that will be not used.
> - I'm working on one more improvement to use nohz_idle_balance in the newly
>    idle case when the system is not overloaded and
>    (this_rq->avg_idle > sysctl_sched_migration_cost). In this case, we can try to
>    use nohz_idle_balance with NOHZ_STATS_KICK and abort as soon as it exceed
>    this_rq->avg_idle. This will remove some calls to kick_ilb and some wake up
>    of an idle cpus.
> 
> ---
>   kernel/sched/fair.c | 72 +++++++++++++++++++++++++----------------------------
>   1 file changed, 34 insertions(+), 38 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 52114c6..898785d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5386,7 +5386,6 @@ static struct {
>   	atomic_t stats_state;
>   	unsigned long next_balance;     /* in jiffy units */
>   	unsigned long next_stats;
> -	struct timer_list timer;
>   } nohz ____cacheline_aligned;
>   
>   #endif /* CONFIG_NO_HZ_COMMON */
> @@ -8004,8 +8003,9 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>   		prefer_sibling = 1;
>   
>   #ifdef CONFIG_NO_HZ_COMMON
> -	if (env->idle == CPU_NEWLY_IDLE)
> +	if (env->idle == CPU_NEWLY_IDLE && atomic_read(&nohz.stats_state)) {
>   		env->flags |= LBF_NOHZ_STATS;
> +	}
>   #endif
>   
>   	load_idx = get_sd_load_idx(env->sd, env->idle);
> @@ -8818,6 +8818,8 @@ update_next_balance(struct sched_domain *sd, unsigned long *next_balance)
>   		*next_balance = next;
>   }
>   
> +static void kick_ilb(unsigned int flags);
> +
>   /*
>    * idle_balance is called by schedule() if this_cpu is about to become
>    * idle. Attempts to pull tasks from other CPUs.
> @@ -8852,12 +8854,16 @@ static int idle_balance(struct rq *this_rq, struct rq_flags *rf)
>   
>   	if (this_rq->avg_idle < sysctl_sched_migration_cost ||
>   	    !this_rq->rd->overload) {
> +		unsigned long next = READ_ONCE(nohz.next_stats);
>   		rcu_read_lock();
>   		sd = rcu_dereference_check_sched_domain(this_rq->sd);
>   		if (sd)
>   			update_next_balance(sd, &next_balance);
>   		rcu_read_unlock();
>   
> +		if (time_after(jiffies, next) && atomic_read(&nohz.stats_state))
> +			kick_ilb(NOHZ_STATS_KICK);
> +
>   		goto out;
>   	}
>   
> @@ -9075,18 +9081,6 @@ static void kick_ilb(unsigned int flags)
>   	smp_send_reschedule(ilb_cpu);
>   }
>   
> -void nohz_balance_timer(struct timer_list *timer)
> -{
> -	unsigned long next = READ_ONCE(nohz.next_stats);
> -
> -	if (time_before(jiffies, next)) {
> -		mod_timer(timer, next);
> -		return;
> -	}
> -
> -	kick_ilb(NOHZ_STATS_KICK);
> -}
> -
>   /*
>    * Current heuristic for kicking the idle load balancer in the presence
>    * of an idle cpu in the system.
> @@ -9122,6 +9116,9 @@ static void nohz_balancer_kick(struct rq *rq)
>   	if (likely(!atomic_read(&nohz.nr_cpus)))
>   		return;
>   
> +	if (time_after(now, nohz.next_stats) && atomic_read(&nohz.stats_state))
> +		flags = NOHZ_STATS_KICK;
> +
>   	if (time_before(now, nohz.next_balance))
>   		goto out;
>   
> @@ -9227,7 +9224,6 @@ static void set_cpu_sd_state_idle(int cpu)
>   void nohz_balance_enter_idle(int cpu)
>   {
>   	struct rq *rq = cpu_rq(cpu);
> -	unsigned int val, new;
>   
>   	SCHED_WARN_ON(cpu != smp_processor_id());
>   
> @@ -9251,6 +9247,7 @@ void nohz_balance_enter_idle(int cpu)
>   		return;
>   
>   	rq->nohz_tick_stopped = 1;
> +	rq->has_blocked_load = 1;
>   
>   	cpumask_set_cpu(cpu, nohz.idle_cpus_mask);
>   	atomic_inc(&nohz.nr_cpus);
> @@ -9258,21 +9255,11 @@ void nohz_balance_enter_idle(int cpu)
>   	set_cpu_sd_state_idle(cpu);
>   
>   	/*
> -	 * implies a barrier such that if the stats_state update is observed
> -	 * the above updates are also visible. Pairs with stuff in
> -	 * update_sd_lb_stats() and nohz_idle_balance().
> +	 * Each time a cpu enter idle, we assume that it has blocked load and
> +	 * enable the periodic update of the load of idle cpus
>   	 */
> -	val = atomic_read(&nohz.stats_state);
> -	do {
> -		new = val + 2;
> -		new |= 1;
> -	} while (!atomic_try_cmpxchg(&nohz.stats_state, &val, new));
> +	atomic_set(&nohz.stats_state, 1);
>   
> -	/*
> -	 * If the timer was stopped, restart the thing.
> -	 */
> -	if (!(val & 1))
> -		mod_timer(&nohz.timer, jiffies + msecs_to_jiffies(LOAD_AVG_PERIOD));
>   }
>   #else
>   static inline void nohz_balancer_kick(struct rq *rq) { }
> @@ -9409,7 +9396,6 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>   	bool has_blocked_load = false;
>   	int update_next_balance = 0;
>   	int this_cpu = this_rq->cpu;
> -	unsigned int stats_seq;
>   	unsigned int flags;
>   	int balance_cpu;
>   	struct rq *rq;
> @@ -9422,7 +9408,6 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>   		return false;
>   	}
>   
> -	stats_seq = atomic_read(&nohz.stats_state);
>   	/*
>   	 * barrier, pairs with nohz_balance_enter_idle(), ensures ...
>   	 */
> @@ -9432,6 +9417,16 @@ 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
> +	 * 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.
> +	 */
> +	atomic_set(&nohz.stats_state, 0);
> +
>   	for_each_cpu(balance_cpu, nohz.idle_cpus_mask) {
>   		if (balance_cpu == this_cpu || !idle_cpu(balance_cpu))
>   			continue;
> @@ -9441,8 +9436,10 @@ 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())
> +		if (need_resched()) {
> +			has_blocked_load = true;
>   			break;
> +		}
>   
>   		rq = cpu_rq(balance_cpu);
>   
> @@ -9477,12 +9474,12 @@ 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);
>   
> -	if (has_blocked_load ||
> -	    !atomic_try_cmpxchg(&nohz.stats_state, &stats_seq, 0)) {
> -		WRITE_ONCE(nohz.next_stats,
> -				now + msecs_to_jiffies(LOAD_AVG_PERIOD));
> -		mod_timer(&nohz.timer, nohz.next_stats);
> -	}
> +	WRITE_ONCE(nohz.next_stats,
> +		now + msecs_to_jiffies(LOAD_AVG_PERIOD));
> +
> +	/* There is still blocked load, enable periodic update */
> +	if (has_blocked_load)
> +		atomic_set(&nohz.stats_state, 1);
>   
>   	/*
>   	 * next_balance will be updated only when there is a need.
> @@ -10115,7 +10112,6 @@ __init void init_sched_fair_class(void)
>   	nohz.next_balance = jiffies;
>   	nohz.next_stats = jiffies;
>   	zalloc_cpumask_var(&nohz.idle_cpus_mask, GFP_NOWAIT);
> -	timer_setup(&nohz.timer, nohz_balance_timer, 0);
>   #endif
>   #endif /* SMP */
>   
> 

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