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Subject[PATCH 2/9] sched/fair: Track efficiency of task recent_used_cpu
Date
This simply tracks the efficiency of the recent_used_cpu. The hit rate
of this matters as it can avoid a domain search. Similarly, the miss
rate matters because each miss is a penalty to the fast path. MMTests
uses this to generate three additional metrics

SIS Recent Used Hit: A recent CPU was eligible and used. Each hit is
a domain search avoided.

SIS Recent Used Miss: A recent CPU was eligible but unavailable. Each
time this is hit, there was a penalty to the fast path before
a domain search happened.

SIS Recent Success Rate: A percentage of the number of hits versus
the total attempts to use the recent CPU.

SIS Recent Attempts: The total number of times the recent CPU was examined.
A high number of Recent Attempts with a low Success Rate implies
the fast path is being punished severely. This could have been
presented as a weighting of hits and misses but calculating an
appropriate weight for misses is problematic.

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
kernel/sched/debug.c | 2 ++
kernel/sched/fair.c | 28 ++++++++++++++++------------
kernel/sched/sched.h | 2 ++
kernel/sched/stats.c | 7 ++++---
4 files changed, 24 insertions(+), 15 deletions(-)

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 603d4bc71612..1ec87b7bb6a9 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -742,6 +742,8 @@ do { \
P(sis_domain_search);
P(sis_scanned);
P(sis_failed);
+ P(sis_recent_hit);
+ P(sis_recent_miss);
}
#undef P

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index cc0b451d1794..4d48dc08a49b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6388,18 +6388,22 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)

/* Check a recently used CPU as a potential idle candidate: */
recent_used_cpu = p->recent_used_cpu;
- if (recent_used_cpu != prev &&
- recent_used_cpu != target &&
- cpus_share_cache(recent_used_cpu, target) &&
- (available_idle_cpu(recent_used_cpu) || sched_idle_cpu(recent_used_cpu)) &&
- cpumask_test_cpu(p->recent_used_cpu, p->cpus_ptr) &&
- asym_fits_capacity(task_util, recent_used_cpu)) {
- /*
- * Replace recent_used_cpu with prev as it is a potential
- * candidate for the next wake:
- */
- p->recent_used_cpu = prev;
- return recent_used_cpu;
+ if (recent_used_cpu != prev && recent_used_cpu != target) {
+
+ if (cpus_share_cache(recent_used_cpu, target) &&
+ (available_idle_cpu(recent_used_cpu) || sched_idle_cpu(recent_used_cpu)) &&
+ cpumask_test_cpu(p->recent_used_cpu, p->cpus_ptr) &&
+ asym_fits_capacity(task_util, recent_used_cpu)) {
+ /*
+ * Replace recent_used_cpu with prev as it is a potential
+ * candidate for the next wake:
+ */
+ schedstat_inc(this_rq()->sis_recent_hit);
+ p->recent_used_cpu = prev;
+ return recent_used_cpu;
+ }
+
+ schedstat_inc(this_rq()->sis_recent_miss);
}

/*
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 4cf307763fe9..1c04d7a97dbe 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1078,6 +1078,8 @@ struct rq {
unsigned int sis_domain_search;
unsigned int sis_scanned;
unsigned int sis_failed;
+ unsigned int sis_recent_hit;
+ unsigned int sis_recent_miss;
#endif

#ifdef CONFIG_CPU_IDLE
diff --git a/kernel/sched/stats.c b/kernel/sched/stats.c
index 7dd9b0dec437..1aa648edb88b 100644
--- a/kernel/sched/stats.c
+++ b/kernel/sched/stats.c
@@ -10,7 +10,7 @@
* Bump this up when changing the output format or the meaning of an existing
* format, so that tools can adapt (or abort)
*/
-#define SCHEDSTAT_VERSION 16
+#define SCHEDSTAT_VERSION 17

static int show_schedstat(struct seq_file *seq, void *v)
{
@@ -30,14 +30,15 @@ static int show_schedstat(struct seq_file *seq, void *v)

/* runqueue-specific stats */
seq_printf(seq,
- "cpu%d %u 0 %u %u %u %u %llu %llu %lu %u %u %u %u",
+ "cpu%d %u 0 %u %u %u %u %llu %llu %lu %u %u %u %u %u %u",
cpu, rq->yld_count,
rq->sched_count, rq->sched_goidle,
rq->ttwu_count, rq->ttwu_local,
rq->rq_cpu_time,
rq->rq_sched_info.run_delay, rq->rq_sched_info.pcount,
rq->sis_search, rq->sis_domain_search,
- rq->sis_scanned, rq->sis_failed);
+ rq->sis_scanned, rq->sis_failed,
+ rq->sis_recent_hit, rq->sis_recent_miss);

seq_printf(seq, "\n");

--
2.26.2
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