linux-kernel - [PATCH 5/5] sched: limit sched_slice if it is more than sysctl_sched

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Date:	Thu, 28 Mar 2013 16:58:56 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, Mike Galbraith <efault@....de>,
	Paul Turner <pjt@...gle.com>, Alex Shi <alex.shi@...el.com>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Namhyung Kim <namhyung@...nel.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: [PATCH 5/5] sched: limit sched_slice if it is more than sysctl_sched_latency

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@....com>

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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