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Date:   Tue, 5 Oct 2021 22:49:27 +0200
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
Subject: Re: [PATCH 2/2] sched/fair: Skip update_blocked_averages if we are
 defering load balance

On Mon, Oct 04, 2021 at 07:14:51PM +0200, Vincent Guittot wrote:
> In newidle_balance(), the scheduler skips load balance to the new idle cpu
> when the 1st sd of this_rq is:
> 
>    this_rq->avg_idle < sd->max_newidle_lb_cost
> 
> Doing a costly call to update_blocked_averages() will not be useful and
> simply adds overhead when this condition is true.
> 
> Check the condition early in newidle_balance() to skip
> update_blocked_averages() when possible.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
> ---
>  kernel/sched/fair.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 1f78b2e3b71c..1294b78503d9 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10841,17 +10841,20 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>  	 */
>  	rq_unpin_lock(this_rq, rf);
>  
> +	rcu_read_lock();
> +	sd = rcu_dereference_check_sched_domain(this_rq->sd);
> +
>  	if (this_rq->avg_idle < sysctl_sched_migration_cost ||
> -	    !READ_ONCE(this_rq->rd->overload)) {
> +		!READ_ONCE(this_rq->rd->overload) ||
> +		(sd && this_rq->avg_idle < sd->max_newidle_lb_cost)) {

set cino=(0:0, please.

Also, people have, in the past, tried to get rid of the first clause
here, perhaps this can replace it instead of augment it?

>  
> -		rcu_read_lock();
> -		sd = rcu_dereference_check_sched_domain(this_rq->sd);
>  		if (sd)
>  			update_next_balance(sd, &next_balance);
>  		rcu_read_unlock();
>  
>  		goto out;
>  	}
> +	rcu_read_unlock();

There's another rcu_read_lock section right below this, at the very
least we can merge them.

Also, IIRC we're running all this with premption disabled, and since
rcu-sched got folded into rcu, all that rcu_read_*lock() stuff isn't
strictly required anymore.

(we're full circle there, back in the day RCU implied RCU-sched and the
scheduler relied on preempt-disable for lots of this stuff, then Paul
split them, and I spend a fair amount of time adding all this
rcu_read_*lock() crud, and now he's merge them again, and it can go
again).

Except of course, I think we need to make rcu_dereference_check happy
first :/

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