Re: [LKP] Re: [mm/memcg] bd0b230fe1: will-it-scale.per_process_ops -22.7% regression

Feng Tang
2020-11-13T07:34:46Z

On Thu, Nov 12, 2020 at 03:16:54PM +0100, Michal Hocko wrote:
> On Thu 12-11-20 20:28:44, Feng Tang wrote:
> > Hi Michal,
> >
> > On Wed, Nov 04, 2020 at 09:15:46AM +0100, Michal Hocko wrote:
> > > > > > Hi Michal,
> > > > > >
> > > > > > We used the default configure of cgroups, not sure what configuration you
> > > > > > want,
> > > > > > could you give me more details? and here is the cgroup info of will-it-scale
> > > > > > process:
> > > > > >
> > > > > > $ cat /proc/3042/cgroup
> > > > > > 12:hugetlb:/
> > > > > > 11:memory:/system.slice/lkp-bootstrap.service
> > > > >
> > > > > OK, this means that memory controler is enabled and in use. Btw. do you
> > > > > get the original performance if you add one phony page_counter after the
> > > > > union?
> > > > >
> > > > I add one phony page_counter after the union and re-test, the regression
> > > > reduced to -1.2%. It looks like the regression caused by the data structure
> > > > layout change.
> > >
> > > Thanks for double checking. Could you try to cache align the
> > > page_counter struct? If that helps then we should figure which counters
> > > acks against each other by adding the alignement between the respective
> > > counters.
> >
> > We tried below patch to make the 'page_counter' aligned.
> >
> > diff --git a/include/linux/page_counter.h b/include/linux/page_counter.h
> > index bab7e57..9efa6f7 100644
> > --- a/include/linux/page_counter.h
> > +++ b/include/linux/page_counter.h
> > @@ -26,7 +26,7 @@ struct page_counter {
> > /* legacy */
> > unsigned long watermark;
> > unsigned long failcnt;
> > -};
> > +} ____cacheline_internodealigned_in_smp;
> >
> > and with it, the -22.7% peformance change turns to a small -1.7%, which
> > confirms the performance bump is caused by the change to data alignment.
> >
> > After the patch, size of 'page_counter' increases from 104 bytes to 128
> > bytes, and the size of 'mem_cgroup' increases from 2880 bytes to 3008
> > bytes(with our kernel config). Another major data structure which
> > contains 'page_counter' is 'hugetlb_cgroup', whose size will change
> > from 912B to 1024B.
> >
> > Should we make these page_counters aligned to reduce cacheline conflict?
>
> I would rather focus on a more effective mem_cgroup layout. It is very
> likely that we are just stumbling over two counters here.
>
> Could you try to add cache alignment of counters after memory and see
> which one makes the difference? I do not expect memsw to be the one
> because that one is used together with the main counter. But who knows
> maybe the way it crosses the cache line has the exact effect. Hard to
> tell without other numbers.

I added some alignments change around the 'memsw', but neither of them can
restore the -22.7%. Following are some log showing how the alignments
are:

tl: memcg=0x7cd1000 memory=0x7cd10d0 memsw=0x7cd1140 kmem=0x7cd11b0 tcpmem=0x7cd1220
t2: memcg=0x7cd0000 memory=0x7cd00d0 memsw=0x7cd0140 kmem=0x7cd01c0 tcpmem=0x7cd0230

So both of the 'memsw' are aligned, but t2's 'kmem' is aligned while
t1's is not.

I will check more on the perf data about detailed hotspots.

Thanks,
Feng

> Btw. it would be great to see what the effect is on cgroup v2 as well.
>
> Thanks for pursuing this!
> --
> Michal Hocko
> SUSE Labs