linux-kernel - Re: [PATCH v2 2/2] sched/fair: Trigger nohz.next_balance updates when a CPU goes NOHZ-idle

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Date:   Mon, 19 Jul 2021 17:24:46 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Valentin Schneider <valentin.schneider@....com>,
        linux-kernel@...r.kernel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH v2 2/2] sched/fair: Trigger nohz.next_balance updates when
 a CPU goes NOHZ-idle

On 19/07/2021 12:31, Valentin Schneider wrote:

[...]

> @@ -10351,6 +10352,9 @@ static void nohz_balancer_kick(struct rq *rq)
>  unlock:
>  	rcu_read_unlock();
>  out:
> +	if (READ_ONCE(nohz.needs_update))
> +		flags |= NOHZ_NEXT_KICK;
> +

Since NOHZ_NEXT_KICK is part of NOHZ_KICK_MASK, some conditions above
will already set it in flags. Is this intended?

>  	if (flags)
>  		kick_ilb(flags);
>  }
> @@ -10447,12 +10451,13 @@ void nohz_balance_enter_idle(int cpu)
>  	/*
>  	 * Ensures that if nohz_idle_balance() fails to observe our
>  	 * @idle_cpus_mask store, it must observe the @has_blocked
> -	 * store.
> +	 * and @needs_update stores.
>  	 */
>  	smp_mb__after_atomic();
>  
>  	set_cpu_sd_state_idle(cpu);
>  
> +	WRITE_ONCE(nohz.needs_update, 1);
>  out:
>  	/*
>  	 * Each time a cpu enter idle, we assume that it has blocked load and
> @@ -10501,13 +10506,17 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags,

function header would need update to incorporate the new 'update
nohz.next_balance' functionality. It only talks about 'update of blocked
load' and 'complete load balance' so far.

>  	/*
>  	 * We assume there will be no idle load after this update and clear
>  	 * the has_blocked flag. If a cpu enters idle in the mean time, it will
> -	 * set the has_blocked flag and trig another update of idle load.
> +	 * set the has_blocked flag and trigger another update of idle load.
>  	 * Because a cpu that becomes idle, is added to idle_cpus_mask before
>  	 * setting the flag, we are sure to not clear the state and not
>  	 * check the load of an idle cpu.
> +	 *
> +	 * Same applies to idle_cpus_mask vs needs_update.
>  	 */
>  	if (flags & NOHZ_STATS_KICK)
>  		WRITE_ONCE(nohz.has_blocked, 0);
> +	if (flags & NOHZ_NEXT_KICK)
> +		WRITE_ONCE(nohz.needs_update, 0);
>  
>  	/*
>  	 * Ensures that if we miss the CPU, we must see the has_blocked
> @@ -10531,6 +10540,8 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
>  		if (need_resched()) {
>  			if (flags & NOHZ_STATS_KICK)
>  				has_blocked_load = true;

This looks weird now? 'has_blocked_load = true' vs
'WRITE_ONCE(nohz.needs_update, 1)'.

> +			if (flags & NOHZ_NEXT_KICK)
> +				WRITE_ONCE(nohz.needs_update, 1);
>  			goto abort;
>  		}
>  
>