Re: [PATCH] kernel/hung_task.c: Monitor killed tasks.

[Dmitry Vyukov]

From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Date: Mon, May 13, 2019 at 1:04 PM
To: Andrew Morton
Cc: Ingo Molnar, Peter Zijlstra, Paul E. McKenney, Petr Mladek, Vitaly
Kuznetsov, Liu Chuansheng, Valdis Kletnieks,
<linux-kernel@xxxxxxxxxxxxxxx>, Tetsuo Handa, Dmitry Vyukov

> syzbot's second top report is "no output from test machine" where the
> userspace process failed to spawn a new test process for 300 seconds
> for some reason. One of reasons which can result in this report is that
> an already spawned test process was unable to terminate (e.g. trapped at
> an unkillable retry loop due to some bug) after SIGKILL was sent to that
> process. Therefore, reporting when a thread is failing to terminate
> despite a fatal signal is pending would give us more useful information.
>
> This version shares existing sysctl settings (e.g. check interval,
> timeout, whether to panic) used for detecting TASK_UNINTERRUPTIBLE
> threads, for I don't know whether people want to use a new kernel
> config option and different sysctl settings for monitoring killed
> threads.
>
> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> ---
>  include/linux/sched.h |  1 +
>  kernel/hung_task.c    | 44 ++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 45 insertions(+)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index a2cd1585..d42bdd7 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -850,6 +850,7 @@ struct task_struct {
>  #ifdef CONFIG_DETECT_HUNG_TASK
>         unsigned long                   last_switch_count;
>         unsigned long                   last_switch_time;
> +       unsigned long                   killed_time;
>  #endif
>         /* Filesystem information: */
>         struct fs_struct                *fs;
> diff --git a/kernel/hung_task.c b/kernel/hung_task.c
> index f108a95..34e7b84 100644
> --- a/kernel/hung_task.c
> +++ b/kernel/hung_task.c
> @@ -141,6 +141,47 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
>         touch_nmi_watchdog();
>  }
>
> +static void check_killed_task(struct task_struct *t, unsigned long timeout)
> +{
> +       unsigned long stamp = t->killed_time;
> +
> +       /*
> +        * Ensure the task is not frozen.
> +        * Also, skip vfork and any other user process that freezer should skip.
> +        */
> +       if (unlikely(t->flags & (PF_FROZEN | PF_FREEZER_SKIP)))
> +               return;
> +       /*
> +        * Skip threads which are already inside do_exit(), for exit_mm() etc.
> +        * might take many seconds.
> +        */
> +       if (t->flags & PF_EXITING)
> +               return;
> +       if (!stamp) {
> +               stamp = jiffies;
> +               if (!stamp)
> +                       stamp++;
> +               t->killed_time = stamp;
> +               return;
> +       }
> +       if (time_is_after_jiffies(stamp + timeout * HZ))
> +               return;
> +       trace_sched_process_hang(t);
> +       if (sysctl_hung_task_panic) {
> +               console_verbose();
> +               hung_task_call_panic = true;
> +       }
> +       /*
> +        * This thread failed to terminate for more than
> +        * sysctl_hung_task_timeout_secs seconds, complain:
> +        */
> +       pr_err("INFO: task %s:%d can't die for more than %ld seconds.\n",
> +              t->comm, t->pid, (jiffies - stamp) / HZ);
> +       sched_show_task(t);
> +       hung_task_show_lock = true;
> +       touch_nmi_watchdog();
> +}
> +
>  /*
>   * To avoid extending the RCU grace period for an unbounded amount of time,
>   * periodically exit the critical section and enter a new one.
> @@ -192,6 +233,9 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
>                                 goto unlock;
>                         last_break = jiffies;
>                 }
> +               /* Check threads which are about to terminate. */
> +               if (unlikely(fatal_signal_pending(t)))
> +                       check_killed_task(t, timeout);
>                 /* use "==" to skip the TASK_KILLABLE tasks waiting on NFS */
>                 if (t->state == TASK_UNINTERRUPTIBLE)
>                         check_hung_task(t, timeout);
> --
> 1.8.3.1


For background context:
syzkaller has found a number of cases with unkillable tasks ([1]),
but they were always poorly diagnosed by the kernel. In all cases
everything just hangs with no additional diagnosis and in all cases
it required manual work just to identify that there is an instance
of an unkillable task in the pile.
With syzbot all such bugs end up in the "no output from test machine" pile ([2])
with hundreds of thousands of crashes, which nobody usually looks at
(except for Tetsuo). Most likely we now have many more of these in the pile
so it would be very useful to make it possible to auto-diagnose and auto-bucket
these bugs.


[1] search for "unkillable" at
https://github.com/google/syzkaller/blob/master/docs/linux/found_bugs.md
[2] https://syzkaller.appspot.com/bug?id=0b210638616bb68109e9642158d4c0072770ae1c