Re: [RFC][PATCH -mm 2/2] Freezer: Use wait queue instead of busy looping

Rafael J. Wysocki
2007-07-31T09:39:23Z

Hi,

On Tuesday, 31 July 2007 10:01, Pavel Machek wrote:
> Hi!
>
> > > > refrigerator_called is only reset after try_to_freeze_tasks() has found it
> > > > equal to one. There is only a small window between checking it in
> > > > wait_event_timeout() and resetting it,
> > >
> > > Yes.
> > >
> > > > but then we go to send freeze requests
> > > > to the remaining tasks and we count 'todo' from the start, so that shouldn't
> > > > be a problem.
> > >
> > > ... and we find the task which is not frozen() yet, but which has already passed
> > > the "set condition and wakeup", increment todo, and wait for the event. If it was
> > > the last task, we will sleep until timeout.
> > >
> > > I agree, this is not fatal and unlikely, but still it is a race. I think it is
> > > better to move this code down, after frozen_process().
> >
> > OK, I see your point. The updated patch is appended.
> >
> > > (offtopic: strictly speaking, we don't even need the "refrigerator_called", we
> > > only need the wait_queue_head_t. try_to_freeze_tasks() just adds the "current"
> > > to wq at the very start of the main loop).
> >
> > Hmm, yes, I think so.
>
> Ok, could we just do schedule_timeout(HZ/10) or something, but when we
> _know_ we woke someone, wakeup() that task? No new variables, keep
> existing logic.

The logic doesn't change that much. :-)

> That should still get most of the benefits, and be two liner, no?

Well, I think we can avoid using refrigerator_called, if this is a problem, but
the patch won't be a two liner.

Greetings,
Rafael

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