[PATCH] sched: initialize runtime to non-zero on cfs bw set

[Vladimir Davydov]

If cfs_rq->runtime_remaining is <= 0 then either
- cfs_rq is throttled and waiting for quota redistribution, or
- cfs_rq is currently executing and will be throttled on
  put_prev_entity, or
- cfs_rq is not throttled and has not executed since its quota was set
  (runtime_remaining is set to 0 on cfs bandwidth reconfiguration).

It is obvious that the last case is rather an exception from the rule
"runtime_remaining<=0 iff cfs_rq is throttled or will be throttled as
soon as it finishes its execution". Moreover, it can lead to a task hang
as follows. If put_prev_task is called immediately after first
pick_next_task after quota was set, "immediately" meaning rq->clock in
both functions is the same, then the corresponding cfs_rq will be
throttled. Besides being unfair (the cfs_rq has not executed in fact),
the quota refilling timer can be idle at that time and it won't be
activated on put_prev_task because update_curr calls
account_cfs_rq_runtime, which activates the timer, only if delta_exec is
strictly positive. As a result we can get a task "running" inside a
throttled cfs_rq which will probably never be unthrottled.

To avoid the problem, the patch makes tg_set_cfs_bandwidth initialize
runtime_remaining of each cfs_rq to 1 instead of 0 so that the cfs_rq
will be throttled only if it has executed for some positive number of
nanoseconds.
--
Several times we had our customers encountered such hangs inside a VM
(seems something is wrong or rather different in time accounting there).
Analyzing crash dumps revealed that hung tasks were running inside
cfs_rq's, which had the following setup

cfs_rq->throttled=1
cfs_rq->runtime_enabled=1
cfs_rq->runtime_remaining=0
cfs_rq->tg->cfs_bandwidth.idle=1
cfs_rq->tg->cfs_bandwidth.timer_active=0

which conforms pretty nice to the explanation given above.

Signed-off-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
---
 kernel/sched/core.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 26058d0..c7a078f 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7686,7 +7686,7 @@ static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota)
 
 		raw_spin_lock_irq(&rq->lock);
 		cfs_rq->runtime_enabled = runtime_enabled;
-		cfs_rq->runtime_remaining = 0;
+		cfs_rq->runtime_remaining = 1;
 
 		if (cfs_rq->throttled)
 			unthrottle_cfs_rq(cfs_rq);
-- 
1.7.1

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