Re: Re: [RFC][PATCH -mm 5/6] Freezer: Use freezing timeout more efficiently

Rafael J. Wysocki
2007-07-10T20:57:57Z

On Tuesday, 10 July 2007 22:35, Oleg Nesterov wrote:
> On 07/10, Rafael J. Wysocki wrote:
> >
> > On Tuesday, 10 July 2007 20:50, Oleg Nesterov wrote:
> > >
> > > I just noticed we don't use thaw_process(), this means that the retry doesn't
> > > play well with wait_event_freezable() introduced in the previous patch.
> > >
> > > Suppose that kthread_stop(T) hangs and blocks the freezer, and T does
> >
> > The retry mechanism only applies to user land processes. :-)
> >
> > > while (!kthread_should_stop()) {
> > > wait_event_freezable(...);
> > > do_something();
> > > }
> > >
> > > and it is freezed. We clear PF_FROZEN but not TIF_FREEZE, wait_event_freezable
> > > checks freezing() and goes to refrigerator again.
> >
> > PF_FROZEN and TIF_FREEZE are mutually exclusive as long as the thawing is
> > not racing with refrigerator().
>
> Ah yes, I forgot. Thanks!
>
> > > Another problem is that we only count UNINTERRUPTIBLE tasks to make a decision
> > > about retry, while wait_event_freezable() sleeps NTERRUPTIBLE.
> >
> > The retry thing doesn't cover kernel threads, because they aren't supposed to
> > block the freezer. They are supposed to freeze voluntarily and to know
> > _exactly_ what they are doing.
>
> Ok, I see... So, the plan is to eventually fix the "problematic" things like
> kthread_stop(), yes?

Yes. Last time I looked at it kthread_stop() was a moving target. ;-)

> Unless I missed something again, we can't reliably
> kthread_stop() the thread above even if it does
>
> wait_event_freezable(my_condition || kthread_should_stop())

Not sure, will check.

Greetings,
Rafael

--
"Premature optimization is the root of all evil." - Donald Knuth