We noticed that with cgroup CPU controller in use, the scheduling
latency gets wonky regardless of nesting level or weight
configuration. This is easily reproducible with Chris Mason's
schbench[1].
All tests are run on a single socket, 16 cores, 32 threads machine.
While the machine is mostly idle, it isn't completey. There's always
some variable management load going on. The command used is
schbench -m 2 -t 16 -s 10000 -c 15000 -r 30
which measures scheduling latency with 32 threads each of which
repeatedly runs for 15ms and then sleeps for 10ms. Here's a
representative result when running from the root cgroup.
# ~/schbench -m 2 -t 16 -s 10000 -c 15000 -r 30
Latency percentiles (usec)
50.0000th: 26
75.0000th: 62
90.0000th: 74
95.0000th: 86
*99.0000th: 887
99.5000th: 3692
99.9000th: 10832
min=0, max=13374
The following is inside a first level CPU cgroup with the maximum
weight.
# ~/schbench -m 2 -t 16 -s 10000 -c 15000 -r 30
Latency percentiles (usec)
50.0000th: 31
75.0000th: 65
90.0000th: 71
95.0000th: 91
*99.0000th: 7288
99.5000th: 10352
99.9000th: 12496
min=0, max=13023
Note the drastic increase in p99 scheduling latency. After
investigation, it turned out that the update_sd_lb_stats(), which is
used by load_balance() to pick the most loaded group, was often
picking the wrong group. A CPU which has one schbench running and
another queued wouldn't report the correspondingly higher
weighted_cpuload() and get looked over as the target of load
balancing.
weighted_cpuload() is the root cfs_rq's runnable_load_avg which is the
sum of the load_avg of all queued sched_entities. Without cgroups or
at the root cgroup, each task's load_avg contributes directly to the
sum. When a task wakes up or goes to sleep, the change is immediately
reflected on runnable_load_avg which in turn affects load balancing.
However, when CPU cgroup is in use, a nesting cfs_rq blocks this
immediate reflection. When a task wakes up inside a cgroup, the
nested cfs_rq's runnable_load_avg is updated but doesn't get
propagated to the parent. It only affects the matching sched_entity's
load_avg over time which then gets propagated to the runnable_load_avg
at that level. This makes weighted_cpuload() often temporarily out of
sync leading to suboptimal behavior of load_balance() and increase in
scheduling latencies as shown above.
This patch fixes the issue by updating propagate_entity_load_avg() to
always propagate to the parent's runnable_load_avg. Combined with the
previous patch, this keeps a cfs_rq's runnable_load_avg always the sum
of the scaled loads of all tasks queued below removing the artifacts
from nesting cfs_rqs. The following is from inside three levels of
nesting with the patch applied.
# ~/schbench -m 2 -t 16 -s 10000 -c 15000 -r 30
Latency percentiles (usec)
50.0000th: 40
75.0000th: 71
90.0000th: 89
95.0000th: 108
*99.0000th: 679
99.5000th: 3500
99.9000th: 10960
min=0, max=13790
[1] git://git.kernel.org/pub/scm/linux/kernel/git/mason/schbench.git
Signed-off-by: Tejun Heo <t...@kernel.org>
Cc: Vincent Guittot <vincent.guit...@linaro.org>
Cc: Ingo Molnar <mi...@redhat.com>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Mike Galbraith <efa...@gmx.de>
Cc: Paul Turner <p...@google.com>
Cc: Chris Mason <c...@fb.com>
---
kernel/sched/fair.c | 34 +++++++++++++++++++++-------------
1 file changed, 21 insertions(+), 13 deletions(-)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3075,7 +3075,8 @@ update_tg_cfs_util(struct cfs_rq *cfs_rq
/* Take into account change of load of a child task group */
static inline void
-update_tg_cfs_load(struct cfs_rq *cfs_rq, struct sched_entity *se)
+update_tg_cfs_load(struct cfs_rq *cfs_rq, struct sched_entity *se,
+ bool propagate_avg)
{
struct cfs_rq *gcfs_rq = group_cfs_rq(se);
long load = 0, delta;
@@ -3113,9 +3114,11 @@ update_tg_cfs_load(struct cfs_rq *cfs_rq
se->avg.load_avg = load;
se->avg.load_sum = se->avg.load_avg * LOAD_AVG_MAX;
- /* Update parent cfs_rq load */
- add_positive(&cfs_rq->avg.load_avg, delta);
- cfs_rq->avg.load_sum = cfs_rq->avg.load_avg * LOAD_AVG_MAX;
+ if (propagate_avg) {
+ /* Update parent cfs_rq load */
+ add_positive(&cfs_rq->avg.load_avg, delta);
+ cfs_rq->avg.load_sum = cfs_rq->avg.load_avg * LOAD_AVG_MAX;
+ }
/*
* If the sched_entity is already enqueued, we also have to update the
@@ -3147,22 +3150,27 @@ static inline int test_and_clear_tg_cfs_
/* Update task and its cfs_rq load average */
static inline int propagate_entity_load_avg(struct sched_entity *se)
{
- struct cfs_rq *cfs_rq;
+ struct cfs_rq *cfs_rq = cfs_rq_of(se);
+ bool propagate_avg;
if (entity_is_task(se))
return 0;
- if (!test_and_clear_tg_cfs_propagate(se))
- return 0;
-
- cfs_rq = cfs_rq_of(se);
+ propagate_avg = test_and_clear_tg_cfs_propagate(se);
- set_tg_cfs_propagate(cfs_rq);
+ /*
+ * We want to keep @cfs_rq->runnable_load_avg always in sync so
+ * that the load balancer can accurately determine the busiest CPU
+ * regardless of cfs_rq nesting.
+ */
+ update_tg_cfs_load(cfs_rq, se, propagate_avg);
- update_tg_cfs_util(cfs_rq, se);
- update_tg_cfs_load(cfs_rq, se);
+ if (propagate_avg) {
+ set_tg_cfs_propagate(cfs_rq);
+ update_tg_cfs_util(cfs_rq, se);
+ }
- return 1;
+ return propagate_avg;
}
#else /* CONFIG_FAIR_GROUP_SCHED */
