On Mon, 7 Dec 2020 at 10:15, Mel Gorman <mgor...@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 <mgor...@techsingularity.net>
Reviewed-by: Vincent Guittot <vincent.guit...@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
>
