linux-kernel - Re: [PATCH v2] kernel/hung

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Date:   Tue, 16 Oct 2018 11:16:34 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH v2] kernel/hung_task.c: disable on suspend

Hi,

Sorry for the delay here.

On Tuesday, September 25, 2018 2:16:36 PM CEST Vitaly Kuznetsov 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
> 
> The root cause of the issue is that under certain circumstances jiffies
> counter keeps advancing, some work to prevent that is currently ongoing.
> However, it seems that it would make sense to disable hung task detector
> on suspend and re-enable it on wakeup regardless.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
> Changes since v1:
> - Implement detector disabling by zeroing timeout [Rafael J. Wysocki]
> ---
>  kernel/hung_task.c | 36 +++++++++++++++++++++++++++++++++++-
>  1 file changed, 35 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/hung_task.c b/kernel/hung_task.c
> index b9132d1269ef..ac6e8c9306bd 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>
> @@ -242,12 +243,14 @@ void reset_hung_task_detector(void)
>  }
>  EXPORT_SYMBOL_GPL(reset_hung_task_detector);
>  
> +static unsigned long hung_last_checked;
> +
>  /*
>   * kthread which checks for tasks stuck in D state
>   */
>  static int watchdog(void *dummy)
>  {
> -	unsigned long hung_last_checked = jiffies;
> +	hung_last_checked = jiffies;
>  
>  	set_user_nice(current, 0);
>  
> @@ -272,9 +275,40 @@ static int watchdog(void *dummy)
>  	return 0;
>  }
>  
> +static int hungtask_pm_notify(struct notifier_block *self,
> +			      unsigned long action, void *hcpu)
> +{
> +	static unsigned long saved_timeout, saved_interval;
> +
> +	switch (action) {
> +	case PM_SUSPEND_PREPARE:
> +	case PM_HIBERNATION_PREPARE:
> +		saved_timeout = sysctl_hung_task_timeout_secs;
> +		saved_interval = sysctl_hung_task_check_interval_secs;
> +		sysctl_hung_task_timeout_secs = 0;
> +		sysctl_hung_task_check_interval_secs = 0;
> +		wake_up_process(watchdog_task);
> +		break;

AFAICS, this is racy (for example, it still is possible for user space
to update the sysctl_* values after you've set them to 0).  That can be
fixed, but I'm not sure it is worth the effort.

Since watchdog() is not expected to run very often in general, I think
I prefer your v1 after all, so please resend it.

Thanks,
Rafael