Re: [PATCH v3 4/5] sched/fair: Remove sysctl_sched_migration_cost condition

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 19/10/2021 14:35, Vincent Guittot wrote:
> With a default value of 500us, sysctl_sched_migration_cost is
> significanlty higher than the cost of load_balance. Remove the

Shouldn't this be rather `load balance cost on the lowest sd`? I assume
here that lb cost stands for sd->max_newidle_lb_cost of the 1st sd.

We still use sysctl_sched_migration_cost as a floor against max_cost
(i.e. lb cost of all sd's) when setting rq->max_idle_balance_cost in
rebalance_domains().

And in the add-on discussion (disabling the call to
nohz_newidle_balance() you mention that sd->max_newidle_lb_cost can be
higher than sysctl_sched_migration_cost (even when default 500us).


> condition and rely on the sd->max_newidle_lb_cost to abort
> newidle_balance.
> 
> Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
>  kernel/sched/fair.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e50fd751e1df..57eae0ebc492 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10895,8 +10895,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *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) ||
> +	if (!READ_ONCE(this_rq->rd->overload) ||
>  	    (sd && this_rq->avg_idle < sd->max_newidle_lb_cost)) {
>  
>  		if (sd)
>