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

[Feng Tang]

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