Re: [PATCH 4/5] sched/fair: Tune down misfit nohz kicks

[Valentin Schneider <valentin.schneider@arm.com>]

Hi,

On 06/02/2019 16:04, Peter Zijlstra wrote:
[...]
>> @@ -9561,6 +9573,14 @@ static void nohz_balancer_kick(struct rq *rq)
> 
> 	sd = rcu_dereference(rq->sd);
> 	if (sd) {
> 		if ((rq->cfs.h_nr_running >= 1) &&
> 		    check_cpu_capacity(rq, sd)) {
> 			flags = NOHZ_KICK_MASK;
> 			goto unlock;
>>  		}
>>  	}
>>  
>> +	sd = rcu_dereference(per_cpu(sd_asym_cpucapacity, cpu));
>> +	if (sd) {
>> +		if (check_misfit_status(rq, sd)) {
>> +			flags = NOHZ_KICK_MASK;
>> +			goto unlock;
>> +		}
>> +	}
> 
> So while the exact @sd to use for check_cpu_capacity() likely doesn't
> matter; this is a 'implicit' test for actually having asym_capacity.
> 

I did feel compelled to use the "right" @sd to have a coherent
imbalance_pct being used - on big.LITTLE, the @sd above this hunk would be
MC (117 imbalance_pct) whereas the sd_asym_cpucapacity one would be
DIE (125 imbalance_pct).

> Fair enough I suppose. However, now that you wrote such a nice comment
> for the sd_llc_shared case, these other two cases are sad to not have a
> comment.
> 
> So how about you add something like:
> 
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9589,8 +9589,12 @@ static void nohz_balancer_kick(struct rq
>  
>  	sd = rcu_dereference(rq->sd);
>  	if (sd) {
> -		if ((rq->cfs.h_nr_running >= 1) &&
> -		    check_cpu_capacity(rq, sd)) {
> +		/*
> +		 * If there's a CFS task and the current CPU has reduced
> +		 * capacity; kick the ILB to see if there's a better CPU to run
> +		 * on.
> +		 */
> +		if (rq->cfs.h_nr_running >= 1 && check_cpu_capacity(rq, sd)) {
>  			flags = NOHZ_KICK_MASK;
>  			goto unlock;
>  		}
> @@ -9598,6 +9602,10 @@ static void nohz_balancer_kick(struct rq
>  
>  	sd = rcu_dereference(per_cpu(sd_asym_cpucapacity, cpu));
>  	if (sd) {
> +		/*
> +		 * When ASYM_CAPACITY; see if there's a higher capacity CPU to
> +		 * run the misfit task on.
> +		 */
>  		if (check_misfit_status(rq, sd)) {
>  			flags = NOHZ_KICK_MASK;
>  			goto unlock;
> @@ -9606,6 +9614,10 @@ static void nohz_balancer_kick(struct rq
>  
>  	sd = rcu_dereference(per_cpu(sd_asym_packing, cpu));
>  	if (sd) {
> +		/*
> +		 * When ASYM_PACKING; see if there's a more preferred CPU going
> +		 * idle; in which case, kick the ILB to move tasks around.
> +		 */

s/going idle/currently idle/, no?

>  		for_each_cpu_and(i, sched_domain_span(sd), nohz.idle_cpus_mask) {
>  			if (sched_asym_prefer(i, cpu)) {
>  				flags = NOHZ_KICK_MASK;
>