[PATCH 07/15] sched,cfs: fix zero length timeslice calculation

[Rik van Riel <riel@surriel.com>]

The way the time slice length is currently calculated, not only do high
priority tasks get longer time slices than low priority tasks, but due
to fixed point math, low priority tasks could end up with a zero length
time slice. This can lead to cache thrashing and other inefficiencies.

Cap the minimum time slice length to sysctl_sched_min_granularity.

Tasks that end up getting a time slice length too long for their relative
priority will simply end up having their vruntime advanced much faster than
other tasks, resulting in them receiving time slices less frequently.

Signed-off-by: Rik van Riel <riel@surriel.com>
---
 kernel/sched/fair.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 31a26737a873..8f8c85c6da9b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -732,6 +732,13 @@ static u64 sched_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
 		}
 		slice = __calc_delta(slice, se->load.weight, load);
 	}
+
+	/*
+	 * To avoid cache thrashing, run at least sysctl_sched_min_granularity.
+	 * The vruntime of a low priority task advances faster; those tasks
+	 * will simply get time slices less frequently.
+	 */
+	slice = max_t(u64, slice, sysctl_sched_min_granularity);
 	return slice;
 }
 
-- 
2.20.1