Re: [mm/writeback] 8d92890bd6: will-it-scale.per_process_ops -15.3% regression

[Jan Kara]

 On Thu 15-10-20 11:08:43, Jan Kara wrote:
> On Thu 15-10-20 08:46:01, NeilBrown wrote:
> > On Wed, Oct 14 2020, Jan Kara wrote:
> > 
> > > On Wed 14-10-20 16:47:06, kernel test robot wrote:
> > >> Greeting,
> > >> 
> > >> FYI, we noticed a -15.3% regression of will-it-scale.per_process_ops due
> > >> to commit:
> > >> 
> > >> commit: 8d92890bd6b8502d6aee4b37430ae6444ade7a8c ("mm/writeback: discard
> > >> NR_UNSTABLE_NFS, use NR_WRITEBACK instead")
> > >> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
> > >
> > > Thanks for report but it doesn't quite make sense to me. If we omit
> > > reporting & NFS changes in that commit (which is code not excercised by
> > > this benchmark), what remains are changes like:
> > >
> > >         nr_pages += node_page_state(pgdat, NR_FILE_DIRTY);
> > > -       nr_pages += node_page_state(pgdat, NR_UNSTABLE_NFS);
> > >         nr_pages += node_page_state(pgdat, NR_WRITEBACK);
> > > ...
> > > -               nr_reclaimable = global_node_page_state(NR_FILE_DIRTY) +
> > > -                                       global_node_page_state(NR_UNSTABLE_NFS);
> > > +               nr_reclaimable = global_node_page_state(NR_FILE_DIRTY);
> > > ...
> > > -       gdtc->dirty = global_node_page_state(NR_FILE_DIRTY) +
> > > -                     global_node_page_state(NR_UNSTABLE_NFS);
> > > +       gdtc->dirty = global_node_page_state(NR_FILE_DIRTY);
> > >
> > > So if there's any negative performance impact of these changes, they're
> > > likely due to code alignment changes or something like that... So I don't
> > > think there's much to do here since optimal code alignment is highly specific
> > > to a particular CPU etc.
> > 
> > I agree, it seems odd.
> > 
> > Removing NR_UNSTABLE_NFS from enum node_stat_item would renumber all the
> > following value and would, I think, change NR_DIRTIED from 32 to 31.
> > Might that move something to a different cache line and change some
> > contention?
> 
> Interesting theory, it could be possible.
> 
> > That would be easy enough to test: just re-add NR_UNSTABLE_NFS.
> 
> Yeah, easy enough to test. Patch for this is attached. 0-day people, can
> you check whether applying this patch changes anything in your perf
> numbers?

Forgot the patch. Attached now.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR