[tip:sched/core] sched: Guarantee new group-entities always have weight

From: tip-bot for Paul Turner
Date: Tue Oct 29 2013 - 10:11:03 EST

Commit-ID: 0ac9b1c21874d2490331233b3242085f8151e166
Gitweb: http://git.kernel.org/tip/0ac9b1c21874d2490331233b3242085f8151e166
Author: Paul Turner <pjt@xxxxxxxxxx>
AuthorDate: Wed, 16 Oct 2013 11:16:27 -0700
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 29 Oct 2013 12:02:23 +0100

sched: Guarantee new group-entities always have weight

Currently, group entity load-weights are initialized to zero. This
admits some races with respect to the first time they are re-weighted in
earlty use. ( Let g[x] denote the se for "g" on cpu "x". )

Suppose that we have root->a and that a enters a throttled state,
immediately followed by a[0]->t1 (the only task running on cpu[0])

put_prev_task(group_cfs_rq(a[0]), t1)
put_prev_entity(..., t1)

Then, before unthrottling occurs, let a[0]->b[0]->t2 wake for the first

enqueue_task_fair(rq[0], t2)
enqueue_entity(group_cfs_rq(b[0]), t2)
enqueue_entity_load_avg(group_cfs_rq(b[0]), t2)
account_entity_enqueue(group_cfs_ra(b[0]), t2)
< skipped because b is part of a throttled hierarchy >
enqueue_entity(group_cfs_rq(a[0]), b[0])

We now have b[0] enqueued, yet group_cfs_rq(a[0])->load.weight == 0
which violates invariants in several code-paths. Eliminate the
possibility of this by initializing group entity weight.

Signed-off-by: Paul Turner <pjt@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/20131016181627.22647.47543.stgit@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
kernel/sched/fair.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f6308cb..0923ab2 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7198,7 +7198,8 @@ void init_tg_cfs_entry(struct task_group *tg, struct cfs_rq *cfs_rq,
se->cfs_rq = parent->my_q;

se->my_q = cfs_rq;
- update_load_set(&se->load, 0);
+ /* guarantee group entities always have weight */
+ update_load_set(&se->load, NICE_0_LOAD);
se->parent = parent;

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