[tip:sched/urgent] sched/deadline: Correctly handle active 0-lag timers

From: tip-bot for luca abeni
Date: Tue Apr 16 2019 - 11:33:24 EST

Commit-ID: 1b02cd6a2d7f3e2a6a5262887d2cb2912083e42f
Gitweb: https://git.kernel.org/tip/1b02cd6a2d7f3e2a6a5262887d2cb2912083e42f
Author: luca abeni <luca.abeni@xxxxxxxxxxxxxxx>
AuthorDate: Mon, 25 Mar 2019 14:15:30 +0100
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 16 Apr 2019 16:54:58 +0200

sched/deadline: Correctly handle active 0-lag timers

syzbot reported the following warning:

[Â] WARNING: CPU: 4 PID: 17089 at kernel/sched/deadline.c:255 task_non_contending+0xae0/0x1950

line 255 of deadline.c is:


in task_non_contending().

Unfortunately, in some cases (for example, a deadline task
continuosly blocking and waking immediately) it can happen that
a task blocks (and task_non_contending() is called) while the
0-lag timer is still active.

In this case, the safest thing to do is to immediately decrease
the running bandwidth of the task, without trying to re-arm the 0-lag timer.

Signed-off-by: luca abeni <luca.abeni@xxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Juri Lelli <juri.lelli@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: chengjian (D) <cj.chengjian@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20190325131530.34706-1-luca.abeni@xxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
kernel/sched/deadline.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 6a73e41a2016..43901fa3f269 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -252,7 +252,6 @@ static void task_non_contending(struct task_struct *p)
if (dl_entity_is_special(dl_se))

- WARN_ON(hrtimer_active(&dl_se->inactive_timer));

zerolag_time = dl_se->deadline -
@@ -269,7 +268,7 @@ static void task_non_contending(struct task_struct *p)
* If the "0-lag time" already passed, decrease the active
* utilization now, instead of starting a timer
- if (zerolag_time < 0) {
+ if ((zerolag_time < 0) || hrtimer_active(&dl_se->inactive_timer)) {
if (dl_task(p))
sub_running_bw(dl_se, dl_rq);
if (!dl_task(p) || p->state == TASK_DEAD) {