Commit 8a99b6833c88 ("sched: Move SCHED_DEBUG sysctl to debugfs")
moved the kernel.sched_wakeup_granularity_ns sysctl under debugfs.
One of the reasons why this sysctl may be used may be for "optimising
for throughput", particularly when overloaded. The tool TuneD sometimes
alters this for two profiles e.g. "mssql" and "throughput-performance". At
least version 2.9 does but it changed in master where it also will poke
at debugfs instead. This patch aims to reduce the motivation to tweak
sysctl_sched_wakeup_granularity by increasing sched_wakeup_granularity
if the running task runtime has not exceeded sysctl_sched_min_granularity.
During task migration or wakeup, a decision is made on whether
to preempt the current task or not. To limit over-scheduled,
sysctl_sched_wakeup_granularity delays the preemption to allow at least 1ms
of runtime before preempting. However, when a domain is heavily overloaded
(e.g. hackbench), the degree of over-scheduling is still severe. This is
problematic as time is wasted rescheduling tasks that could instead be
used by userspace tasks.
However, care must be taken. Even if a system is overloaded, there may
be high priority threads that must still be able to run. Mike Galbraith
explained the constraints as follows;
CFS came about because the O1 scheduler was unfair to the
point it had starvation problems. People pretty much across the
board agreed that a fair scheduler was a much way better way
to go, and CFS was born. It didn't originally have the sleep
credit business, but had to grow it to become _short term_ fair.
Ingo cut the sleep credit in half because of overscheduling, and
that has worked out pretty well all told.. but now you're pushing
it more in the unfair direction, all the way to extremely unfair
for anything and everything very light.
Fairness isn't the holy grail mind you, and at some point, giving
up on short term fairness certainly isn't crazy, as proven by your
hackbench numbers and other numbers we've seen over the years,
but taking bites out of the 'CF' in the CFS that was born to be a
corner-case killer is.. worrisome. The other shoe will drop.. it
always does :)
This patch increases the wakeup granularity if the current task has not
reached its minimum preemption granularity. The current task may still
be preempted but the difference in runtime must be higher.
hackbench-process-pipes
5.15.0-rc3 5.15.0-rc3
sched-wakeeflips-v1r1sched-scalewakegran-v3r2
Amean 1 0.3890 ( 0.00%) 0.3823 ( 1.71%)
Amean 4 0.5217 ( 0.00%) 0.4867 ( 6.71%)
Amean 7 0.5387 ( 0.00%) 0.5053 ( 6.19%)
Amean 12 0.5443 ( 0.00%) 0.5450 ( -0.12%)
Amean 21 0.6487 ( 0.00%) 0.6807 ( -4.93%)
Amean 30 0.8033 ( 0.00%) 0.7107 * 11.54%*
Amean 48 1.2400 ( 0.00%) 1.0447 * 15.75%*
Amean 79 1.8200 ( 0.00%) 1.6033 * 11.90%*
Amean 110 2.5820 ( 0.00%) 2.0763 * 19.58%*
Amean 141 3.2203 ( 0.00%) 2.5313 * 21.40%*
Amean 172 3.8200 ( 0.00%) 3.1163 * 18.42%*
Amean 203 4.3357 ( 0.00%) 3.5560 * 17.98%*
Amean 234 4.8047 ( 0.00%) 3.8913 * 19.01%*
Amean 265 5.1243 ( 0.00%) 4.2293 * 17.47%*
Amean 296 5.5940 ( 0.00%) 4.5357 * 18.92%*
5.15.0-rc3 5.15.0-rc3
sched-wakeeflips-v1r1 sched-scalewakegran-v3r2
Duration User 2567.27 2034.17
Duration System 21098.79 17137.08
Duration Elapsed 136.49 120.2
Signed-off-by: Mel Gorman <[email protected]>
Reviewed-by: Vincent Guittot <[email protected]>
---
kernel/sched/fair.c | 17 +++++++++++++++--
kernel/sched/features.h | 2 ++
2 files changed, 17 insertions(+), 2 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6e476f6d9435..b07ac726e011 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7012,10 +7012,23 @@ balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
}
#endif /* CONFIG_SMP */
-static unsigned long wakeup_gran(struct sched_entity *se)
+static unsigned long
+wakeup_gran(struct sched_entity *curr, struct sched_entity *se)
{
unsigned long gran = sysctl_sched_wakeup_granularity;
+ if (sched_feat(SCALE_WAKEUP_GRAN)) {
+ unsigned long delta_exec;
+
+ /*
+ * Increase the wakeup granularity if curr's runtime
+ * is less than the minimum preemption granularity.
+ */
+ delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
+ if (delta_exec < sysctl_sched_min_granularity)
+ gran += sysctl_sched_min_granularity;
+ }
+
/*
* Since its curr running now, convert the gran from real-time
* to virtual-time in his units.
@@ -7054,7 +7067,7 @@ wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se)
if (vdiff <= 0)
return -1;
- gran = wakeup_gran(se);
+ gran = wakeup_gran(curr, se);
if (vdiff > gran)
return 1;
diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index 1cf435bbcd9c..7b70a409cfa5 100644
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -100,3 +100,5 @@ SCHED_FEAT(LATENCY_WARN, false)
SCHED_FEAT(ALT_PERIOD, true)
SCHED_FEAT(BASE_SLICE, true)
+
+SCHED_FEAT(SCALE_WAKEUP_GRAN, true)
--
2.31.1