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

Rafael J. Wysocki
2007-07-31T10:08:40Z

On Tuesday, 31 July 2007 11:39, Rafael J. Wysocki wrote:
> 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.

To be precise, we'd need to add current to the wait queue manually, which
would require us to open code wait_event_timeout(), more or less.

Still, maybe to many things are done in this patch at a time. I'll try to
split it into smaller steps. :-)

Greetings,
Rafael

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