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

Feng Tang
2020-11-20T11:44:32Z

On Fri, Nov 13, 2020 at 03:34:36PM +0800, Feng Tang wrote:
> On Thu, Nov 12, 2020 at 03:16:54PM +0100, Michal Hocko wrote:
> > > > > 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.

Some more check updates about it:

Waiman's patch is effectively removing one 'struct page_counter' between
'memory' and "memsw'. And the mem_cgroup is:

struct mem_cgroup {

...

struct page_counter memory; /* Both v1 & v2 */

union {
struct page_counter swap; /* v2 only */
struct page_counter memsw; /* v1 only */
};

/* Legacy consumer-oriented counters */
struct page_counter kmem; /* v1 only */
struct page_counter tcpmem; /* v1 only */

...
...

MEMCG_PADDING(_pad1_);

atomic_t moving_account;
struct task_struct *move_lock_task;

...
};

I do experiments by inserting a 'page_counter' between 'memory'
and the 'MEMCG_PADDING(_pad1_)', no matter where I put it, the
benchmark result can be recovered from 145K to 185K, which is
really confusing, as adding a 'page_counter' right before the
'_pad1_' doesn't change cache alignment of any members.

Thanks,
Feng