Tracing showed that the per-cpu scanning cost of __select_idle_core() (~120ns) was significantly higher than that of __select_idle_cpu() (~40ns). This means that, even when reduced to the minimal scan, we're still 3x more expensive than the simple search. perf annotate suggested this was mostly due to cache-misses on the additional cpumasks used. However, we can mitigate this by only doing the more expensive search when there is a good chance it is beneficial. After all, when there are no idle cores to be had, there's no point in looking for any (SMT>2 might want to try without this). Clearing has_idle_cores early (without an exhaustive search) should be fine because we're eager to set it when a core goes idle again. FOLD 1: 0.568188455 seconds time elapsed ( +- 0.40% ) 2: 0.643264625 seconds time elapsed ( +- 1.27% ) 5: 2.385378263 seconds time elapsed ( +- 1.12% ) 10: 3.808555491 seconds time elapsed ( +- 1.46% ) 20: 6.431994272 seconds time elapsed ( +- 1.21% ) 40: 9.423539507 seconds time elapsed ( +- 2.07% ) FOLD+ 1: 0.554694881 seconds time elapsed ( +- 0.42% ) 2: 0.632730119 seconds time elapsed ( +- 1.84% ) 5: 2.230432464 seconds time elapsed ( +- 1.17% ) 10: 3.549957778 seconds time elapsed ( +- 1.55% ) 20: 6.118364255 seconds time elapsed ( +- 0.72% ) 40: 9.515406550 seconds time elapsed ( +- 1.74% ) Signed-off-by: Peter Zijlstra (Intel) --- kernel/sched/fair.c | 5 ++++- kernel/sched/features.h | 2 +- 2 files changed, 5 insertions(+), 2 deletions(-) --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -6382,6 +6382,8 @@ static int __select_idle_core(struct tas } } + set_idle_cores(target, 0); + return best_cpu; } @@ -6477,7 +6479,8 @@ static int select_idle_cpu(struct task_s time = local_clock(); #ifdef CONFIG_SCHED_SMT - if (sched_feat(SIS_FOLD) && static_branch_likely(&sched_smt_present)) + if (sched_feat(SIS_FOLD) && static_branch_likely(&sched_smt_present) && + test_idle_cores(target, false)) cpu = __select_idle_core(p, sd, target, nr, &loops); else #endif --- a/kernel/sched/features.h +++ b/kernel/sched/features.h @@ -60,7 +60,7 @@ SCHED_FEAT(SIS_PROP, true) SCHED_FEAT(SIS_AGE, true) SCHED_FEAT(SIS_ONCE, true) -SCHED_FEAT(SIS_FOLD, false) +SCHED_FEAT(SIS_FOLD, true) /* * Issue a WARN when we do multiple update_rq_clock() calls