Messages in this thread Patch in this message | | | From | "Joel Fernandes (Google)" <> | Subject | [PATCH] sched/fair: Rate limit calls to update_blocked_averages() for NOHZ | Date | Fri, 22 Jan 2021 10:46:00 -0500 |
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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@kernel.org> Cc: Frederic Weisbecker <fweisbec@gmail.com> Suggested-by: Dietmar Eggeman <dietmar.eggemann@arm.com> Co-developed-by: Qais Yousef <qais.yousef@arm.com> Signed-off-by: Qais Yousef <qais.yousef@arm.com> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
--- 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
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