Re: [PATCH RFC] mm: Implement balance_dirty_pages() through waiting for flusher thread

[Wu Fengguang <fengguang.wu@intel.com>]

On Mon, Jun 21, 2010 at 03:42:39PM +0200, Jan Kara wrote:
> On Fri 18-06-10 12:21:37, Peter Zijlstra wrote:
> > On Thu, 2010-06-17 at 20:04 +0200, Jan Kara wrote:
> > > +               if (bdi_stat(bdi, BDI_WRITTEN) >= bdi->wb_written_head)
> > > +                       bdi_wakeup_writers(bdi); 
> > 
> > For the paranoid amongst us you could make wb_written_head s64 and write
> > the above as:
> > 
> >   if (bdi_stat(bdi, BDI_WRITTEN) - bdi->wb_written_head > 0)
> > 
> > Which, if you assume both are monotonic and wb_written_head is always
> > within 2^63 of the actual bdi_stat() value, should give the same end
> > result and deal with wrap-around.
> > 
> > For when we manage to create a device that can write 2^64 pages in our
> > uptime :-)
>   OK, the fix is simple enough so I've changed it, although I'm not
> paranoic enough ;) (I actually did the math before writing that test).

a bit more change :)

type:

-       u64 wb_written_head
+       s64 wb_written_head

resetting:

-                       bdi->wb_written_head = ~(u64)0;
+                       bdi->wb_written_head = 0;

setting:

                bdi->wb_written_head = bdi_stat(bdi, BDI_WRITTEN) + wc->written;
+               bdi->wb_written_head |= 1;

testing:

        if (bdi->wb_written_head &&
            bdi_stat(bdi, BDI_WRITTEN) - bdi->wb_written_head > 0)

This avoids calling into bdi_wakeup_writers() pointlessly when no one
is being throttled (which is the normal case).

Thanks,
Fengguang