Re: [PATCH 4/9] writeback: track if we're sleeping on progress in balance_dirty_pages()

[Jens Axboe]

On 04/13/2016 07:08 AM, Jan Kara wrote:
> On Wed 30-03-16 09:07:52, Jens Axboe wrote:
> > Note in the bdi_writeback structure if a task is currently being
> > limited in balance_dirty_pages(), waiting for writeback to
> > proceed.
> ...
> > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> > index 11ff8f758631..15e696bc5d14 100644
> > --- a/mm/page-writeback.c
> > +++ b/mm/page-writeback.c
> > @@ -1746,7 +1746,9 @@ pause:
> >   					  pause,
> >   					  start_time);
> >   		__set_current_state(TASK_KILLABLE);
> > +		wb->dirty_sleeping = 1;
> >   		io_schedule_timeout(pause);
> > +		wb->dirty_sleeping = 0;
>
> Huh, but wb->dirty_sleeping is shared by all the processes in the system.
> So this is seriously racy, isn't it? You rather need a counter for this to
> work.

Sure, but it's not _that_ important. It's like wb->dirty_exceeded, we 
have an equally relaxed relationship.

I don't mind making it more solid, but I can't make it a counter without 
making it atomic. Which is why I left it as just a basic assignment. But 
I guess since we only fiddle with it when going to sleep, we can make it 
an atomic and not have to worry about the potential impact.

--
Jens Axboe