Date:   Wed, 25 Nov 2020 14:24:45 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Xing Zhengjun <zhengjun.xing@...ux.intel.com>,
        Waiman Long <longman@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Chris Down <chris@...isdown.name>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>, Tejun Heo <tj@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Yafang Shao <laoar.shao@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        lkp@...el.com, zhengjun.xing@...el.com, ying.huang@...el.com,
        andi.kleen@...el.com
Subject: Re: [LKP] Re: [mm/memcg] bd0b230fe1: will-it-scale.per_process_ops
 -22.7% regression

On Fri, Nov 20, 2020 at 07:44:24PM +0800, Feng Tang wrote:
> On Fri, Nov 13, 2020 at 03:34:36PM +0800, Feng Tang wrote:
> > > 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.

I think we finally found the trick :), further debugging shows it
is not related to the alignment inside one cacheline, but the
adjacency of 2 adjacent cacheliens (2N and 2N+1, one pair of 128 bytes).

For structure mem_cgroup, member 'vmstats_local', 'vmstats_percpu'
sit in one cacheline, while 'vmstats[]' sits in the next cacheline,
and when 'adjacent cacheline prefetch" is enabled, if these 2 lines
sit in one pair (128 btyes), say 2N and 2N+1, then there seems to
be some kind of false sharing, and if they sit in 2 pairs, say
2N-1 and 2N then it's fine.

And with the following patch to relayout these members, the regression
is restored and event better. while reducing 64 bytes of sizeof
'struct mem_cgroup'

	parent_commit	Waiman's_commit		+relayout patch

result	187K		145K			200K

Also, if we disable the hw prefetch feature, the Waiman's commit
and its parent commit will have no performance difference.

Thanks,
Feng

>From 2e63af34fa4853b2dd9669867c37a3cf07f7a505 Mon Sep 17 00:00:00 2001
From: Feng Tang <feng.tang@...el.com>
Date: Wed, 25 Nov 2020 13:22:21 +0800
Subject: [PATCH] mm: memcg: relayout structure mem_cgroup to avoid cache
 interfereing

0day reported one -22.7% regression for will-it-scale page_fault2
case [1] on a 4 sockets 144 CPU platform, and bisected to it to be
caused by Waiman's optimization (commit bd0b230fe1) of saving one
'struct page_counter' space for 'struct mem_cgroup'.

Initially we thought it was due to the cache alignment change introduced
by the patch, but further debug shows that it is due to some hot data
members ('vmstats_local', 'vmstats_percpu', 'vmstats') sit in 2 adjacent
cacheline (2N and 2N+1 cacheline), and when adjacent cache line prefetch
is enabled, it triggers an "extended level" of cache false sharing for
2 adjacent cache lines.

So exchange the 2 member blocks, while keeping mostly the original
cache alignment, which can restore and even enhance the performance,
and save 64 bytes of space for 'struct mem_cgroup' (from 2880 to 2816,
with 0day's default RHEL-8.3 kernel config)

[1]. https://lore.kernel.org/lkml/20201102091543.GM31092@shao2-debian/

Fixes: bd0b230fe145 ("mm/memcg: unify swap and memsw page counters")
Reported-by: kernel test robot <rong.a.chen@...el.com>
Signed-off-by: Feng Tang <feng.tang@...el.com>
---
 include/linux/memcontrol.h | 28 ++++++++++++++--------------
 1 file changed, 14 insertions(+), 14 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index e391e3c..a2d50b0 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -282,20 +282,6 @@ struct mem_cgroup {
 
 	MEMCG_PADDING(_pad1_);
 
-	/*
-	 * set > 0 if pages under this cgroup are moving to other cgroup.
-	 */
-	atomic_t		moving_account;
-	struct task_struct	*move_lock_task;
-
-	/* Legacy local VM stats and events */
-	struct memcg_vmstats_percpu __percpu *vmstats_local;
-
-	/* Subtree VM stats and events (batched updates) */
-	struct memcg_vmstats_percpu __percpu *vmstats_percpu;
-
-	MEMCG_PADDING(_pad2_);
-
 	atomic_long_t		vmstats[MEMCG_NR_STAT];
 	atomic_long_t		vmevents[NR_VM_EVENT_ITEMS];
 
@@ -317,6 +303,20 @@ struct mem_cgroup {
 	struct list_head objcg_list; /* list of inherited objcgs */
 #endif
 
+	MEMCG_PADDING(_pad2_);
+
+	/*
+	 * set > 0 if pages under this cgroup are moving to other cgroup.
+	 */
+	atomic_t		moving_account;
+	struct task_struct	*move_lock_task;
+
+	/* Legacy local VM stats and events */
+	struct memcg_vmstats_percpu __percpu *vmstats_local;
+
+	/* Subtree VM stats and events (batched updates) */
+	struct memcg_vmstats_percpu __percpu *vmstats_percpu;
+
 #ifdef CONFIG_CGROUP_WRITEBACK
 	struct list_head cgwb_list;
 	struct wb_domain cgwb_domain;
-- 
2.7.4

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