Note: This is a bad idea, it's for illustration only to show how the search space can be filtered at each stage. Searching an idle_cpu_mask would be a potential option. select_idle_core() would be left alone as it has its own throttling mechanism
select_idle_core() may search a full domain for an idle core even if idle CPUs exist result in an excessive search. This patch partially limits the search for an idle core similar to select_idle_cpu() once an idle candidate is found. Note that this patch can *increase* the number of runqueues considered. Any searching done by select_idle_core() is duplicated by select_idle_cpu() if an idle candidate is not found. If there is an idle CPU then aborting select_idle_core() can have a negative impact. This is addressed in the next patch. Signed-off-by: Mel Gorman <mgor...@techsingularity.net> --- kernel/sched/fair.c | 16 +++++++++++++--- 1 file changed, 13 insertions(+), 3 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 33ce65b67381..cd95daf9f53e 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -6095,7 +6095,8 @@ void __update_idle_core(struct rq *rq) * there are no idle cores left in the system; tracked through * sd_llc->shared->has_idle_cores and enabled through update_idle_core() above. */ -static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int target) +static int select_idle_core(struct task_struct *p, struct sched_domain *sd, + int target, int nr) { int idle_candidate = -1; struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask); @@ -6115,6 +6116,11 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int for_each_cpu(cpu, cpu_smt_mask(core)) { schedstat_inc(this_rq()->sis_scanned); + + /* Apply limits if there is an idle candidate */ + if (idle_candidate != -1) + nr--; + if (!available_idle_cpu(cpu)) { idle = false; if (idle_candidate != -1) @@ -6130,6 +6136,9 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int if (idle) return core; + if (!nr) + break; + cpumask_andnot(cpus, cpus, cpu_smt_mask(core)); } @@ -6165,7 +6174,8 @@ static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int t #else /* CONFIG_SCHED_SMT */ -static inline int select_idle_core(struct task_struct *p, struct sched_domain *sd, int target) +static inline int select_idle_core(struct task_struct *p, struct sched_domain *sd, + int target, int nr) { return -1; } @@ -6349,7 +6359,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target) depth = sis_search_depth(sd, this_sd); schedstat_inc(this_rq()->sis_domain_search); - i = select_idle_core(p, sd, target); + i = select_idle_core(p, sd, target, depth); if ((unsigned)i < nr_cpumask_bits) return i; -- 2.26.2