[PATCH] sched/fair: Rate limit calls to update_blocked_averages() for NOHZ

[Joel Fernandes (Google)]

On an octacore ARM64 device running ChromeOS Linux kernel v5.4, I found
that there are a lot of calls to update_blocked_averages(). This causes
the schedule loop to slow down to taking upto 500 micro seconds at
times (due to newidle load balance). I have also seen this manifest in
the periodic balancer.

Closer look shows that the problem is caused by the following
ingredients:
1. If the system has a lot of inactive CGroups (thanks Dietmar for
suggesting to inspect /proc/sched_debug for this), this can make
__update_blocked_fair() take a long time.

2. The device has a lot of CPUs in a cluster which causes schedutil in a
shared frequency domain configuration to be slower than usual. (the load
average updates also try to update the frequency in schedutil).

3. The CPU is running at a low frequency causing the scheduler/schedutil
code paths to take longer than when running at a high CPU frequency.

The fix is simply rate limit the calls to update_blocked_averages to 20
times per second. It appears that updating the blocked average less
often is sufficient. Currently I see about 200 calls per second
sometimes, which seems overkill.

schbench shows a clear improvement with the change:

Without patch:
~/schbench -t 2 -m 2 -r 5
Latency percentiles (usec) runtime 5 (s) (212 total samples)
        50.0th: 210 (106 samples)
        75.0th: 619 (53 samples)
        90.0th: 665 (32 samples)
        95.0th: 703 (11 samples)
        *99.0th: 12656 (8 samples)
        99.5th: 12784 (1 samples)
        99.9th: 13424 (1 samples)
        min=15, max=13424

With patch:
~/schbench -t 2 -m 2 -r 5
Latency percentiles (usec) runtime 5 (s) (214 total samples)
        50.0th: 188 (108 samples)
        75.0th: 238 (53 samples)
        90.0th: 623 (32 samples)
        95.0th: 657 (12 samples)
        *99.0th: 717 (7 samples)
        99.5th: 725 (2 samples)
        99.9th: 725 (0 samples)

Cc: Paul McKenney <paulmck@xxxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Suggested-by: Dietmar Eggeman <dietmar.eggemann@xxxxxxx>
Co-developed-by: Qais Yousef <qais.yousef@xxxxxxx>
Signed-off-by: Qais Yousef <qais.yousef@xxxxxxx>
Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 04a3ce20da67..fe2dc0024db5 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8381,7 +8381,7 @@ static bool update_nohz_stats(struct rq *rq, bool force)
 	if (!cpumask_test_cpu(cpu, nohz.idle_cpus_mask))
 		return false;
 
-	if (!force && !time_after(jiffies, rq->last_blocked_load_update_tick))
+	if (!force && !time_after(jiffies, rq->last_blocked_load_update_tick + (HZ/20)))
 		return true;
 
 	update_blocked_averages(cpu);
-- 
2.30.0.280.ga3ce27912f-goog