| Message ID | 20201207091516.24683-4-mgorman@techsingularity.net (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Reduce worst-case scanning of runqueues in select_idle_sibling | expand |
On Mon, 7 Dec 2020 at 10:15, Mel Gorman <mgorman@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@techsingularity.net> Reviewed-by: Vincent Guittot <vincent.guittot@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 >
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; } /*
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@techsingularity.net> --- kernel/sched/fair.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-)