linux-kernel - Re: [PATCH 1/2] sched/fair: skip newidle update stats

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Message-ID: <YY56RBQR912S6ScC@hirez.programming.kicks-ass.net>
Date:   Fri, 12 Nov 2021 15:29:24 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...hat.com, juri.lelli@...hat.com, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, linux-kernel@...r.kernel.org,
        tim.c.chen@...ux.intel.com, joel@...lfernandes.org
Subject: Re: [PATCH 1/2] sched/fair: skip newidle update stats

On Fri, Nov 12, 2021 at 10:58:56AM +0100, Vincent Guittot wrote:
> In case we skip the newly idle LB entirely or we abort it because we are
> going to exceed the avg_idle, we have to make sure to not start an update
> of the blocked load when entering idle
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>  kernel/sched/fair.c | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 13950beb01a2..a162b0ec8963 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10861,7 +10861,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>  	int this_cpu = this_rq->cpu;
>  	u64 t0, t1, curr_cost = 0;
>  	struct sched_domain *sd;
> -	int pulled_task = 0;
> +	int pulled_task = 0, early_stop = 0;
>  
>  	update_misfit_status(NULL, this_rq);
>  
> @@ -10898,8 +10898,16 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>  	if (!READ_ONCE(this_rq->rd->overload) ||
>  	    (sd && this_rq->avg_idle < sd->max_newidle_lb_cost)) {
>  
> -		if (sd)
> +		if (sd) {
>  			update_next_balance(sd, &next_balance);
> +
> +			/*
> +			 * We skip new idle LB because there is not enough
> +			 * time before next wake up. Make sure that we will
> +			 * not kick NOHZ_NEWILB_KICK
> +			 */
> +			early_stop = 1;
> +		}
>  		rcu_read_unlock();
>  
>  		goto out;
> @@ -10918,8 +10926,10 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>  
>  		update_next_balance(sd, &next_balance);
>  
> -		if (this_rq->avg_idle < curr_cost + sd->max_newidle_lb_cost)
> +		if (this_rq->avg_idle < curr_cost + sd->max_newidle_lb_cost) {
> +			early_stop = 1;
>  			break;
> +		}
>  
>  		if (sd->flags & SD_BALANCE_NEWIDLE) {
>  
> @@ -10969,7 +10979,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>  
>  	if (pulled_task)
>  		this_rq->idle_stamp = 0;
> -	else
> +	else if (!early_stop)
>  		nohz_newidle_balance(this_rq);
>  
>  	rq_repin_lock(this_rq, rf);

Urgh code flow is a mess... Let me see if I can fix some of that.

Anyway, does nohz_newidle_balance() want to loose it's ->avg_idle test
with this on?