Messages in this thread Patch in this message | | | From | Mel Gorman <> | Subject | [PATCH 04/10] sched/fair: Return an idle cpu if one is found after a failed search for an idle core | Date | Thu, 3 Dec 2020 14:11:18 +0000 |
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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%*
In this particular test, the cost/benefit is marginal except for 64 which was a point where the machine was over 50% busy but not fully utilised.
Signed-off-by: Mel Gorman <mgorman@techsingularity.net> --- kernel/sched/fair.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index fc48cc99b03d..845bc0cd9158 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; @@ -6084,7 +6085,13 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int schedstat_inc(this_rq()->sis_scanned); if (!available_idle_cpu(cpu)) { idle = false; - break; + if (idle_candidate != -1) + break; + } + + if (idle_candidate == -1 && + cpumask_test_cpu(cpu, p->cpus_ptr)) { + idle_candidate = cpu; } } @@ -6099,7 +6106,7 @@ 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
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