Commit f07333bf authored Jan 21, 2011 by Paul Turner Committed by Ingo Molnar Jan 26, 2011

sched: Avoid expensive initial update_cfs_load()



Since cfs->{load_stamp,load_last} are zero-initalized the initial load update
will consider the delta to be 'since the beginning of time'.

This results in a lot of pointless divisions to bring this large period to be
within the sysctl_sched_shares_window.

Fix this by initializing load_stamp to be 1 at cfs_rq initialization, this
allows for an initial load_stamp > load_last which then lets standard idle
truncation proceed.

We avoid spinning (and slightly improve consistency) by fixing delta to be
[period - 1] in this path resulting in a slightly more predictable shares ramp.
(Previously the amount of idle time preserved by the overflow would range between
[period/2,period-1].)

Signed-off-by: Paul Turner <pjt@google.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20110122044852.102126037@google.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

parent 6d5ab293

kernel/sched.c

+2 −0

Original line number	Diff line number	Diff line
		@@ -7796,6 +7796,8 @@ static void init_cfs_rq(struct cfs_rq cfs_rq, struct rq rq)
		INIT_LIST_HEAD(&cfs_rq->tasks);
		#ifdef CONFIG_FAIR_GROUP_SCHED
		cfs_rq->rq = rq;
		/* allow initial update_cfs_load() to truncate */
		cfs_rq->load_stamp = 1;
		#endif
		cfs_rq->min_vruntime = (u64)(-(1LL << 20));
		}

kernel/sched_fair.c

+1 −0

Original line number	Diff line number	Diff line
		@@ -733,6 +733,7 @@ static void update_cfs_load(struct cfs_rq *cfs_rq, int global_update)
		now - cfs_rq->load_last > 4 * period) {
		cfs_rq->load_period = 0;
		cfs_rq->load_avg = 0;
		delta = period - 1;
		}

		cfs_rq->load_stamp = now;

Admin message