[PATCH 1/1] sched/fair: ratelimit update to tg->load_avg

[Aaron Lu <aaron.lu@intel.com>]

When using sysbench to benchmark Postgres in a single docker instance
with sysbench's nr_threads set to nr_cpu, it is observed there are times
update_cfs_group() and update_load_avg() shows noticeable overhead on
a 2sockets/112core/224cpu Intel Sapphire Rapids(SPR):

    13.75%    13.74%  [kernel.vmlinux]           [k] update_cfs_group
    10.63%    10.04%  [kernel.vmlinux]           [k] update_load_avg

Annotate shows the cycles are mostly spent on accessing tg->load_avg
with update_load_avg() being the write side and update_cfs_group() being
the read side. tg->load_avg is per task group and when different tasks
of the same taskgroup running on different CPUs frequently access
tg->load_avg, it can be heavily contended.

E.g. when running postgres_sysbench on a 2sockets/112cores/224cpus Intel
Sappire Rapids, during a 5s window, the wakeup number is 14millions and
migration number is 11millions and with each migration, the task's load
will transfer from src cfs_rq to target cfs_rq and each change involves
an update to tg->load_avg. Since the workload can trigger as many wakeups
and migrations, the access(both read and write) to tg->load_avg can be
unbound. As a result, the two mentioned functions showed noticeable
overhead. With netperf/nr_client=nr_cpu/UDP_RR, the problem is worse:
during a 5s window, wakeup number is 21millions and migration number is
14millions; update_cfs_group() costs ~25% and update_load_avg() costs ~16%.

Reduce the overhead by limiting updates to tg->load_avg to at most once
per ms. After this change, the cost of accessing tg->load_avg is greatly
reduced and performance improved. Detailed test results below.

==============================
postgres_sysbench on SPR:
25%
base:   42382±19.8%
patch:  50174±9.5%  (noise)

50%
base:   67626±1.3%
patch:  67365±3.1%  (noise)

75%
base:   100216±1.2%
patch:  112470±0.1% +12.2%

100%
base:    93671±0.4%
patch:  113563±0.2% +21.2%

==============================
hackbench on ICL:
group=1
base:    114912±5.2%
patch:   117857±2.5%  (noise)

group=4
base:    359902±1.6%
patch:   361685±2.7%  (noise)

group=8
base:    461070±0.8%
patch:   491713±0.3% +6.6%

group=16
base:    309032±5.0%
patch:   378337±1.3% +22.4%

=============================
hackbench on SPR:
group=1
base:    100768±2.9%
patch:   103134±2.9%  (noise)

group=4
base:    413830±12.5%
patch:   378660±16.6% (noise)

group=8
base:    436124±0.6%
patch:   490787±3.2% +12.5%

group=16
base:    457730±3.2%
patch:   680452±1.3% +48.8%

============================
netperf/udp_rr on ICL
25%
base:    114413±0.1%
patch:   115111±0.0% +0.6%

50%
base:    86803±0.5%
patch:   86611±0.0%  (noise)

75%
base:    35959±5.3%
patch:   49801±0.6% +38.5%

100%
base:    61951±6.4%
patch:   70224±0.8% +13.4%

===========================
netperf/udp_rr on SPR
25%
base:   104954±1.3%
patch:  107312±2.8%  (noise)

50%
base:    55394±4.6%
patch:   54940±7.4%  (noise)

75%
base:    13779±3.1%
patch:   36105±1.1% +162%

100%
base:     9703±3.7%
patch:   28011±0.2% +189%

==============================================
netperf/tcp_stream on ICL (all in noise range)
25%
base:    43092±0.1%
patch:   42891±0.5%

50%
base:    19278±14.9%
patch:   22369±7.2%

75%
base:    16822±3.0%
patch:   17086±2.3%

100%
base:    18216±0.6%
patch:   18078±2.9%

===============================================
netperf/tcp_stream on SPR (all in noise range)
25%
base:    34491±0.3%
patch:   34886±0.5%

50%
base:    19278±14.9%
patch:   22369±7.2%

75%
base:    16822±3.0%
patch:   17086±2.3%

100%
base:    18216±0.6%
patch:   18078±2.9%

Reported-by: Nitin Tekchandani <nitin.tekchandani@intel.com>
Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
---
 kernel/sched/fair.c  | 13 ++++++++++++-
 kernel/sched/sched.h |  1 +
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c28206499a3d..a5462d1fcc48 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3664,7 +3664,8 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)
  */
 static inline void update_tg_load_avg(struct cfs_rq *cfs_rq)
 {
-	long delta = cfs_rq->avg.load_avg - cfs_rq->tg_load_avg_contrib;
+	long delta;
+	u64 now;
 
 	/*
 	 * No need to update load_avg for root_task_group as it is not used.
@@ -3672,9 +3673,19 @@ static inline void update_tg_load_avg(struct cfs_rq *cfs_rq)
 	if (cfs_rq->tg == &root_task_group)
 		return;
 
+	/*
+	 * For migration heavy workload, access to tg->load_avg can be
+	 * unbound. Limit the update rate to at most once per ms.
+	 */
+	now = sched_clock_cpu(cpu_of(rq_of(cfs_rq)));
+	if (now - cfs_rq->last_update_tg_load_avg < NSEC_PER_MSEC)
+		return;
+
+	delta = cfs_rq->avg.load_avg - cfs_rq->tg_load_avg_contrib;
 	if (abs(delta) > cfs_rq->tg_load_avg_contrib / 64) {
 		atomic_long_add(delta, &cfs_rq->tg->load_avg);
 		cfs_rq->tg_load_avg_contrib = cfs_rq->avg.load_avg;
+		cfs_rq->last_update_tg_load_avg = now;
 	}
 }
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 6a8b7b9ed089..52ee7027def9 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -593,6 +593,7 @@ struct cfs_rq {
 	} removed;
 
 #ifdef CONFIG_FAIR_GROUP_SCHED
+	u64			last_update_tg_load_avg;
 	unsigned long		tg_load_avg_contrib;
 	long			propagate;
 	long			prop_runnable_sum;
-- 
2.41.0