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Subject[PATCH 3/3] sched: fix cgroup movement of waking process
There is a small race between try_to_wake_up() and sched_move_task(), which is trying
to move the process being woken up.

try_to_wake_up() on CPU0 sched_move_task() on CPU1
--------------------------------+---------------------------------
raw_spin_lock_irqsave(p->pi_lock)
task_waking_fair()
->p.se.vruntime -= cfs_rq->min_vruntime
ttwu_queue()
->send reschedule IPI to another CPU1
raw_spin_unlock_irqsave(p->pi_lock)
task_rq_lock()
-> tring to aquire both p->pi_lock and rq->lock
with IRQ disabled
task_move_group_fair()
->p.se.vruntime -= (old)cfs_rq->min_vruntime
->p.se.vruntime += (new)cfs_rq->min_vruntime
task_rq_unlock()
(via IPI)
sched_ttwu_pending()
raw_spin_lock(rq->lock)
ttwu_do_activate()
...
enqueue_entity()
child.se->vruntime += cfs_rq->min_vruntime
raw_spin_unlock(rq->lock)

As a result, vruntime of the process becomes far bigger than min_vruntime,
if (new)cfs_rq->min_vruntime >> (old)cfs_rq->min_vruntime.
This patch fixes this problem by just ignoring such process in task_move_group_fair(),
because the vruntime has already been normalized in task_waking_fair().

Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
---
kernel/sched_fair.c | 4 ++--
1 files changed, 2 insertions(+), 2 deletions(-)
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index bdaa4ab..3feb3a2 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -4925,10 +4925,10 @@ static void task_move_group_fair(struct task_struct *p, int on_rq)
* to another cgroup's rq. This does somewhat interfere with the
* fair sleeper stuff for the first placement, but who cares.
*/
- if (!on_rq && p->state != TASK_RUNNING)
+ if (!on_rq && p->state != TASK_RUNNING && p->state != TASK_WAKING)
p->se.vruntime -= cfs_rq_of(&p->se)->min_vruntime;
set_task_rq(p, task_cpu(p));
- if (!on_rq && p->state != TASK_RUNNING)
+ if (!on_rq && p->state != TASK_RUNNING && p->state != TASK_WAKING)
p->se.vruntime += cfs_rq_of(&p->se)->min_vruntime;
}
#endif
--
1.7.1


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