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 <[email protected]>
> ---
> 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
