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

[Waiman Long]

On 11/12/20 9:16 AM, 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.
>
> Btw. it would be great to see what the effect is on cgroup v2 as well.
>
> Thanks for pursuing this!

The contention may be in the page counters themselves or it can be in 
other fields below the page counters. The cacheline alignment will cause 
"high_work" just after the page counters to start at a cacheline 
boundary. I will try removing the cacheline alignment in the page 
counter and add it to high_work to see there is any change in 
performance. If there is no change, the performance problem will not be 
in the page counters.

Cheers,
Longman