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Subject[tip:sched/urgent] sched/deadline: Fix switched_from_dl() warning
Commit-ID:  e117cb52bdb4d376b711bee34af6434c9e314b3b
Gitweb: https://git.kernel.org/tip/e117cb52bdb4d376b711bee34af6434c9e314b3b
Author: Juri Lelli <juri.lelli@redhat.com>
AuthorDate: Wed, 11 Jul 2018 09:29:48 +0200
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Sun, 15 Jul 2018 23:47:33 +0200

sched/deadline: Fix switched_from_dl() warning

Mark noticed that syzkaller is able to reliably trigger the following warning:

dl_rq->running_bw > dl_rq->this_bw
WARNING: CPU: 1 PID: 153 at kernel/sched/deadline.c:124 switched_from_dl+0x454/0x608
Kernel panic - not syncing: panic_on_warn set ...

CPU: 1 PID: 153 Comm: syz-executor253 Not tainted 4.18.0-rc3+ #29
Hardware name: linux,dummy-virt (DT)
Call trace:
dump_backtrace+0x0/0x458
show_stack+0x20/0x30
dump_stack+0x180/0x250
panic+0x2dc/0x4ec
__warn_printk+0x0/0x150
report_bug+0x228/0x2d8
bug_handler+0xa0/0x1a0
brk_handler+0x2f0/0x568
do_debug_exception+0x1bc/0x5d0
el1_dbg+0x18/0x78
switched_from_dl+0x454/0x608
__sched_setscheduler+0x8cc/0x2018
sys_sched_setattr+0x340/0x758
el0_svc_naked+0x30/0x34

syzkaller reproducer runs a bunch of threads that constantly switch
between DEADLINE and NORMAL classes while interacting through futexes.

The splat above is caused by the fact that if a DEADLINE task is setattr
back to NORMAL while in non_contending state (blocked on a futex -
inactive timer armed), its contribution to running_bw is not removed
before sub_rq_bw() gets called (!task_on_rq_queued() branch) and the
latter sees running_bw > this_bw.

Fix it by removing a task contribution from running_bw if the task is
not queued and in non_contending state while switched to a different
class.

Reported-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Daniel Bristot de Oliveira <bristot@redhat.com>
Reviewed-by: Luca Abeni <luca.abeni@santannapisa.it>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: claudio@evidence.eu.com
Cc: rostedt@goodmis.org
Link: http://lkml.kernel.org/r/20180711072948.27061-1-juri.lelli@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
kernel/sched/deadline.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index fbfc3f1d368a..10c7b51c0d1f 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -2290,8 +2290,17 @@ static void switched_from_dl(struct rq *rq, struct task_struct *p)
if (task_on_rq_queued(p) && p->dl.dl_runtime)
task_non_contending(p);

- if (!task_on_rq_queued(p))
+ if (!task_on_rq_queued(p)) {
+ /*
+ * Inactive timer is armed. However, p is leaving DEADLINE and
+ * might migrate away from this rq while continuing to run on
+ * some other class. We need to remove its contribution from
+ * this rq running_bw now, or sub_rq_bw (below) will complain.
+ */
+ if (p->dl.dl_non_contending)
+ sub_running_bw(&p->dl, &rq->dl);
sub_rq_bw(&p->dl, &rq->dl);
+ }

/*
* We cannot use inactive_task_timer() to invoke sub_running_bw()
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