[PATCH 09/10] sched/fair: Limit the search for an idle core

[Mel Gorman]

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