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

[Waiman Long <longman@redhat.com>]

On 11/25/20 1:24 AM, Feng Tang wrote:
> 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

Finally, misery solved? Well done.

Your patch looks good. It reduces structure size and improves performance.

Acked-by: Waiman Long <longman@redhat.com>

>  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;