Re: [PATCH v3 1/7] sched: limit cpu search in select_idle_cpu

[Parth Shah <parth@linux.ibm.com>]

On 6/27/19 6:59 AM, subhra mazumdar wrote:
> Put upper and lower limit on cpu search of select_idle_cpu. The lower limit
> is amount of cpus in a core while upper limit is twice that. This ensures
> for any architecture we will usually search beyond a core. The upper limit
> also helps in keeping the search cost low and constant.
> 
> Signed-off-by: subhra mazumdar <subhra.mazumdar@oracle.com>
> ---
>  kernel/sched/fair.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index f35930f..b58f08f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6188,7 +6188,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>  	u64 avg_cost, avg_idle;
>  	u64 time, cost;
>  	s64 delta;
> -	int cpu, nr = INT_MAX;
> +	int cpu, limit, floor, nr = INT_MAX;
> 
>  	this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
>  	if (!this_sd)
> @@ -6206,10 +6206,17 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
> 
>  	if (sched_feat(SIS_PROP)) {
>  		u64 span_avg = sd->span_weight * avg_idle;
> -		if (span_avg > 4*avg_cost)
> +		floor = cpumask_weight(topology_sibling_cpumask(target));
> +		if (floor < 2)
> +			floor = 2;
> +		limit = floor << 1;

Is upper limit an experimental value only or it has any arch specific significance?
Because, AFAIU, systems like POWER9 might have benefit for searching for 4-cores
due to its different cache model. So it can be tuned for arch specific builds then.

Also variable names can be changed for better readability.
floor -> weight_clamp_min
limit -> weight_clamp_max
or something similar


> +		if (span_avg > floor*avg_cost) {
>  			nr = div_u64(span_avg, avg_cost);
> -		else
> -			nr = 4;
> +			if (nr > limit)
> +				nr = limit;
> +		} else {
> +			nr = floor;
> +		}
>  	}
> 
>  	time = local_clock();
> 


Best,
Parth