linux-kernel - Re: [PATCH v2] mm: page_counter: relayout structure to reduce false sharing

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Date:   Wed, 20 Jan 2021 08:56:01 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Feng Tang <feng.tang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>,
        Shakeel Butt <shakeelb@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm: page_counter: relayout structure to reduce false
 sharing

On Tue 19-01-21 15:20:14, Feng Tang wrote:
> When checking a memory cgroup related performance regression [1],
> from the perf c2c profiling data, we found high false sharing for
> accessing 'usage' and 'parent'.
> 
> On 64 bit system, the 'usage' and 'parent' are close to each other,
> and easy to be in one cacheline (for cacheline size == 64+ B). 'usage'
> is usally written, while 'parent' is usually read as the cgroup's
> hierarchical counting nature.
> 
> So move the 'parent' to the end of the structure to make sure they
> are in different cache lines.
> 
> Following are some performance data with the patch, against
> v5.11-rc1. [ In the data, A means a platform with 2 sockets 48C/96T,
> B is a platform of 4 sockests 72C/144T, and if a %stddev will be
> shown bigger than 2%, P100/P50 means number of test tasks equals
> to 100%/50% of nr_cpu]
> 
> will-it-scale/malloc1
> ---------------------
> 	   v5.11-rc1			v5.11-rc1+patch
> 
> A-P100	     15782 ±  2%      -0.1%      15765 ±  3%  will-it-scale.per_process_ops
> A-P50	     21511            +8.9%      23432        will-it-scale.per_process_ops
> B-P100	      9155            +2.2%       9357        will-it-scale.per_process_ops
> B-P50	     10967            +7.1%      11751 ±  2%  will-it-scale.per_process_ops
> 
> will-it-scale/pagefault2
> ------------------------
> 	   v5.11-rc1			v5.11-rc1+patch
> 
> A-P100	     79028            +3.0%      81411        will-it-scale.per_process_ops
> A-P50	    183960 ±  2%      +4.4%     192078 ±  2%  will-it-scale.per_process_ops
> B-P100	     85966            +9.9%      94467 ±  3%  will-it-scale.per_process_ops
> B-P50	    198195            +9.8%     217526        will-it-scale.per_process_ops
> 
> fio (4k/1M is block size)
> -------------------------
> 	   v5.11-rc1			v5.11-rc1+patch
> 
> A-P50-r-4k     16881 ±  2%    +1.2%      17081 ±  2%  fio.read_bw_MBps
> A-P50-w-4k      3931          +4.5%       4111 ±  2%  fio.write_bw_MBps
> A-P50-r-1M     15178          -0.2%      15154        fio.read_bw_MBps
> A-P50-w-1M      3924          +0.1%       3929        fio.write_bw_MBps

Thanks for making results easier to read and understand.

> [1].https://lore.kernel.org/lkml/20201102091543.GM31092@shao2-debian/
> Signed-off-by: Feng Tang <feng.tang@...el.com>
> Reviewed-by: Roman Gushchin <guro@...com>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Michal Hocko <mhocko@...e.com>

Acked-by: Michal Hocko <mhocko@...e.com>

Thanks!

> ---
> Changelogs:
>   
>   v2:
>   * Adjust the format of performance data to be more readable,
>     as suggested by Michal Hocko
> 
>  include/linux/page_counter.h | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/page_counter.h b/include/linux/page_counter.h
> index 85bd413..6795913 100644
> --- a/include/linux/page_counter.h
> +++ b/include/linux/page_counter.h
> @@ -12,7 +12,6 @@ struct page_counter {
>  	unsigned long low;
>  	unsigned long high;
>  	unsigned long max;
> -	struct page_counter *parent;
>  
>  	/* effective memory.min and memory.min usage tracking */
>  	unsigned long emin;
> @@ -27,6 +26,14 @@ struct page_counter {
>  	/* legacy */
>  	unsigned long watermark;
>  	unsigned long failcnt;
> +
> +	/*
> +	 * 'parent' is placed here to be far from 'usage' to reduce
> +	 * cache false sharing, as 'usage' is written mostly while
> +	 * parent is frequently read for cgroup's hierarchical
> +	 * counting nature.
> +	 */
> +	struct page_counter *parent;
>  };
>  
>  #if BITS_PER_LONG == 32
> -- 
> 2.7.4
> 

-- 
Michal Hocko
SUSE Labs