Re: [PATCH 12/12] bdi: use bdi_stat_sum() for more accurate debugfs stats

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

On Wed, Nov 18, 2009 at 06:38:49PM +0800, Peter Zijlstra wrote:
> On Wed, 2009-11-18 at 16:27 +0800, Wu Fengguang wrote:
> > plain text document attachment (bdi-debug-dump-sum.patch)
> > CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> > ---
> >  mm/backing-dev.c |    4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > --- linux.orig/mm/backing-dev.c	2009-11-18 16:25:28.000000000 +0800
> > +++ linux/mm/backing-dev.c	2009-11-18 16:26:10.000000000 +0800
> > @@ -104,8 +104,8 @@ static int bdi_debug_stats_show(struct s
> >  		   "wb_mask:          %8lx\n"
> >  		   "wb_list:          %8u\n"
> >  		   "wb_cnt:           %8u\n",
> > -		   (unsigned long) K(bdi_stat(bdi, BDI_WRITEBACK)),
> > -		   (unsigned long) K(bdi_stat(bdi, BDI_RECLAIMABLE)),
> > +		   (unsigned long) K(bdi_stat_sum(bdi, BDI_WRITEBACK)),
> > +		   (unsigned long) K(bdi_stat_sum(bdi, BDI_RECLAIMABLE)),
> >  		   K(bdi_thresh), K(dirty_thresh),
> >  		   K(background_thresh), nr_wb, nr_dirty, nr_io, nr_more_io,
> >  		   !list_empty(&bdi->bdi_list), bdi->state, bdi->wb_mask,
> > 
> 
> Is this really important? This patch is basically a local DoS for large
> machines.

I did this patch after seeing inaccurate exported numbers,
it may be confusing..

> Imagine someone doing:
> 
> while :; do cat /debug/bdi/*/stats; done
> 
> on a 512 cpu box.

Yes there will be overheads. However it's always possible to
create local DoS with some other kind of busy loop?

Thanks,
Fengguang