[PATCH 5/5] sched: limit sched_slice if it is more than sysctl_sched_latency

[Joonsoo Kim]

sched_slice() compute ideal runtime slice. If there are many tasks
in cfs_rq, period for this cfs_rq is extended to guarantee that each task
has time slice at least, sched_min_granularity. And then each task get
a portion of this period for it. If there is a task which have much larger
load weight than others, a portion of period can exceed far more than
sysctl_sched_latency.

For exampple, you can simply imagine that one task with nice -20 and
9 tasks with nice 0 on one cfs_rq. In this case, load weight sum for
this cfs_rq is 88761 + 9 * 1024, 97977. So a portion of slice for the
task with nice -20 is sysctl_sched_min_granularity * 10 * (88761 / 97977),
that is, approximately, sysctl_sched_min_granularity * 9. This aspect
can be much larger if there is more tasks with nice 0.

So we should limit this possible weird situation.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e232421..6ceffbc 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -645,6 +645,9 @@ static u64 sched_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
 	}
 	slice = calc_delta_mine(slice, se->load.weight, load);
 
+	if (unlikely(slice > sysctl_sched_latency))
+		slice = sysctl_sched_latency;
+
 	return slice;
 }
 
-- 
1.7.9.5

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