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

[Oleg Nesterov <oleg@redhat.com>]

On 09/14, Vitaly Kuznetsov wrote:
>
> "Rafael J. Wysocki" <rjw@rjwysocki.net> writes:
> 
> > On Thursday, September 13, 2018 6:08:51 PM CEST Vitaly Kuznetsov wrote:
> ...
> 
> >> +static int hungtask_pm_notify(struct notifier_block *self,
> >> +			      unsigned long action, void *hcpu)
> >> +{
> >> +	switch (action) {
> >> +	case PM_SUSPEND_PREPARE:
> >> +	case PM_HIBERNATION_PREPARE:
> >> +		hung_detector_suspended = true;
> >> +		break;
> >> +	case PM_POST_SUSPEND:
> >> +	case PM_POST_HIBERNATION:
> >> +		hung_detector_suspended = false;
> >> +		break;
> >> +	default:
> >> +		break;
> >> +	}
> >> +	return NOTIFY_OK;
> >> +}
> >> +
> >>  /*
> >>   * kthread which checks for tasks stuck in D state
> >>   */
> >> @@ -261,7 +282,8 @@ static int watchdog(void *dummy)
> >>  		interval = min_t(unsigned long, interval, timeout);
> >>  		t = hung_timeout_jiffies(hung_last_checked, interval);
> >
> > Since you are adding the notifier anyway, what about designing it to make
> > the thread wait on _PREPARE until the notifier kicks it again on exit
> > fron suspend/hibernation?

Well. I agree that freezable kthreads are not nice, but it seems you are
going to add another questionable interface ;)

Vitaly, could you please update the changelog to explain in details whats
going on?

Where does the caller of pm_suspend() sleep in D state? Why it sleeps more
than 120 seconds?

And. given that it takes system_transition_mutex anyway, can't it use
lock_system_sleep() which marks the caller as PF_FREEZER_SKIP (checked
in check_hung_task()) ?

I have to admit I got lost...

> We can either park the kthread (kthread_park/unpark)

No, no, please don't. Nobody outside of smpboot.c should use this (and
this interface should be reworked). Yes, there are already abused, but
please don't add new users.

Oleg.