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

Feng Tang
2020-11-25T06:25:13Z

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@intel.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@intel.com>
Signed-off-by: Feng Tang <feng.tang@intel.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