[patch 5/5] sched: avoid expensive initial update_cfs_load()

Paul Turner
2011-01-22T05:05:07Z

[-- Attachment #1: sched-fix_first_update_load.patch --]
[-- Type: text/plain, Size: 1648 bytes --]

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>

---
kernel/sched.c | 2 ++
kernel/sched_fair.c | 1 +
2 files changed, 3 insertions(+)

Index: tip3/kernel/sched.c
===================================================================
--- tip3.orig/kernel/sched.c
+++ tip3/kernel/sched.c
@@ -7796,6 +7796,8 @@ static void init_cfs_rq(struct cfs_rq *c
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));
}
Index: tip3/kernel/sched_fair.c
===================================================================
--- tip3.orig/kernel/sched_fair.c
+++ tip3/kernel/sched_fair.c
@@ -732,6 +732,7 @@ static void update_cfs_load(struct cfs_r
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;