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Date:   Fri, 20 Nov 2020 14:19:44 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Feng Tang <feng.tang@intel.com>
Cc:     Xing Zhengjun <zhengjun.xing@linux.intel.com>,
        Waiman Long <longman@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Shakeel Butt <shakeelb@google.com>,
        Chris Down <chris@chrisdown.name>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <guro@fb.com>, Tejun Heo <tj@kernel.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Yafang Shao <laoar.shao@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>, lkp@lists.01.org,
        lkp@intel.com, zhengjun.xing@intel.com, ying.huang@intel.com
Subject: Re: [LKP] Re: [mm/memcg] bd0b230fe1: will-it-scale.per_process_ops
 -22.7% regression
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On Fri 20-11-20 19:44:24, Feng Tang wrote:
> 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.

Have you checked the result of pahole before and after your modification
whether something stands out?

Btw. is this reproducible an different CPU models?
-- 
Michal Hocko
SUSE Labs