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

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

On Thu, Nov 19, 2009 at 04:12:04PM +0800, Peter Zijlstra wrote:
> On Thu, 2009-11-19 at 15:59 +0800, Wu Fengguang wrote:
> > 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?
> 
> Preferably not of the kind that renders your box unusable, esp by unpriv
> users.
> 
> But yes there are probably a few local DoS' around, but I don't think
> that warrants adding another one.

OK I'll drop this patch. I'm not feeling strong on either way.

Andrew/Jens, do you prefer for me to repost the non-fs-specific patches?

Thanks,
Fengguang