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SubjectRe: [PATCH] sched/deadline: Fix switched_from_dl
On Wed, 11 Jul 2018 09:29:48 +0200
Juri Lelli <juri.lelli@redhat.com> wrote:

> Mark noticed that syzkaller is able to reliably trigger the following
>
> 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>
> ---
> 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()

Looking at this code:

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()
* at the 0-lag time, because the task could have been migrated
* while SCHED_OTHER in the meanwhile.
*/
if (p->dl.dl_non_contending)
p->dl.dl_non_contending = 0;

Question. Is the "dl_non_contending" only able to be set
if !task_on_rq_queued(p) is true? In that case, we could just clear it
in the first if block. If it's not true, I would think the subtraction
is needed regardless.

-- Steve

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