[PATCH 3/4] sched/fair: Return an idle cpu if one is found after a failed search for an idle core

[Vincent Guittot]

On Mon, 7 Dec 2020 at 10:15, Mel Gorman <mgorman at techsingularity.net> wrote:
>
> select_idle_core is called when SMT is active and there is likely a free
> core available. It may find idle CPUs but this information is simply
> discarded and the scan starts over again with select_idle_cpu.
>
> This patch caches information on idle CPUs found during the search for
> a core and uses one if no core is found. This is a tradeoff. There may
> be a slight impact when utilisation is low and an idle core can be
> found quickly. It provides improvements as the number of busy CPUs
> approaches 50% of the domain size when SMT is enabled.
>
> With tbench on a 2-socket CascadeLake machine, 80 logical CPUs, HT enabled
>
>                           5.10.0-rc6             5.10.0-rc6
>                            schedstat          idlecandidate
> Hmean     1        500.06 (   0.00%)      505.67 *   1.12%*
> Hmean     2        975.90 (   0.00%)      974.06 *  -0.19%*
> Hmean     4       1902.95 (   0.00%)     1904.43 *   0.08%*
> Hmean     8       3761.73 (   0.00%)     3721.02 *  -1.08%*
> Hmean     16      6713.93 (   0.00%)     6769.17 *   0.82%*
> Hmean     32     10435.31 (   0.00%)    10312.58 *  -1.18%*
> Hmean     64     12325.51 (   0.00%)    13792.01 *  11.90%*
> Hmean     128    21225.21 (   0.00%)    20963.44 *  -1.23%*
> Hmean     256    20532.83 (   0.00%)    20335.62 *  -0.96%*
> Hmean     320    20334.81 (   0.00%)    20147.25 *  -0.92%*
>
> Note that there is a significant corner case. As the SMT scan may be
> terminated early, not all CPUs have been visited and select_idle_cpu()
> is still called for a full scan. This case is handled in the next
> patch.
>
> Signed-off-by: Mel Gorman <mgorman at techsingularity.net>

Reviewed-by: Vincent Guittot <vincent.guittot at linaro.org>

> ---
>  kernel/sched/fair.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 01b38fc17bca..00c3b526a5bd 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6066,6 +6066,7 @@ void __update_idle_core(struct rq *rq)
>   */
>  static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int target)
>  {
> +       int idle_candidate = -1;
>         struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
>         int core, cpu;
>
> @@ -6085,6 +6086,11 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int
>                                 idle = false;
>                                 break;
>                         }
> +
> +                       if (idle_candidate == -1 &&
> +                           cpumask_test_cpu(cpu, p->cpus_ptr)) {
> +                               idle_candidate = cpu;
> +                       }
>                 }
>
>                 if (idle)
> @@ -6098,7 +6104,7 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int
>          */
>         set_idle_cores(target, 0);
>
> -       return -1;
> +       return idle_candidate;
>  }
>
>  /*
> --
> 2.26.2
>