Re: [PATCH RFC] kernel/hung_task.c: disable on suspend

[Rafael J. Wysocki]

On Wed, Sep 12, 2018 at 6:11 PM Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
>
> It is possible to observe hung_task complaints when system goes to
> suspend-to-idle state:
>
>  PM: Syncing filesystems ... done.
>  Freezing user space processes ... (elapsed 0.001 seconds) done.
>  OOM killer disabled.
>  Freezing remaining freezable tasks ... (elapsed 0.002 seconds) done.
>  sd 0:0:0:0: [sda] Synchronizing SCSI cache
>  INFO: task bash:1569 blocked for more than 120 seconds.
>        Not tainted 4.19.0-rc3_+ #687
>  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>  bash            D    0  1569    604 0x00000000
>  Call Trace:
>   ? __schedule+0x1fe/0x7e0
>   schedule+0x28/0x80
>   suspend_devices_and_enter+0x4ac/0x750
>   pm_suspend+0x2c0/0x310

This actually is a good catch, but the problem is related to what
happens to the monotonic clock during suspend to idle.

The clock issue needs to be addressed anyway IMO and then this problem
will go away automatically.

> Register a PM notifier to disable the detector on suspend and re-enable
> back on wakeup.
>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> ---
> RFC: It really makes me wonder why nobody reported this before, makes
>  me think I'm missing something.
> ---
>  kernel/hung_task.c | 26 +++++++++++++++++++++++++-
>  1 file changed, 25 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/hung_task.c b/kernel/hung_task.c
> index b9132d1269ef..d75f288c016f 100644
> --- a/kernel/hung_task.c
> +++ b/kernel/hung_task.c
> @@ -15,6 +15,7 @@
>  #include <linux/lockdep.h>
>  #include <linux/export.h>
>  #include <linux/sysctl.h>
> +#include <linux/suspend.h>
>  #include <linux/utsname.h>
>  #include <linux/sched/signal.h>
>  #include <linux/sched/debug.h>
> @@ -242,6 +243,24 @@ void reset_hung_task_detector(void)
>  }
>  EXPORT_SYMBOL_GPL(reset_hung_task_detector);
>
> +static bool hung_detector_suspended;
> +
> +static int hungtask_pm_notify(struct notifier_block *self,
> +                             unsigned long action, void *hcpu)
> +{
> +       switch (action) {
> +       case PM_SUSPEND_PREPARE:

You'd want PM_HIBERNATION_PREPARE here too I think.

> +               hung_detector_suspended = true;
> +               break;
> +       case PM_POST_SUSPEND:

And PM_POST_HIBERNATION here for consistency.

> +               hung_detector_suspended = false;
> +               break;
> +       default:
> +               break;
> +       }
> +       return NOTIFY_OK;
> +}
> +
>  /*
>   * kthread which checks for tasks stuck in D state
>   */
> @@ -261,7 +280,8 @@ static int watchdog(void *dummy)
>                 interval = min_t(unsigned long, interval, timeout);
>                 t = hung_timeout_jiffies(hung_last_checked, interval);
>                 if (t <= 0) {
> -                       if (!atomic_xchg(&reset_hung_task, 0))
> +                       if (!atomic_xchg(&reset_hung_task, 0) &&
> +                           !hung_detector_suspended)
>                                 check_hung_uninterruptible_tasks(timeout);
>                         hung_last_checked = jiffies;
>                         continue;
> @@ -275,6 +295,10 @@ static int watchdog(void *dummy)
>  static int __init hung_task_init(void)
>  {
>         atomic_notifier_chain_register(&panic_notifier_list, &panic_block);
> +
> +       /* Disable hung task detector on suspend */
> +       pm_notifier(hungtask_pm_notify, 0);
> +
>         watchdog_task = kthread_run(watchdog, NULL, "khungtaskd");
>
>         return 0;
> --
> 2.14.4
>