linux-kernel - Re: [PATCH 1/2] mm: page_counter: relayout structure to reduce false sharing

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Date:   Mon, 4 Jan 2021 14:03:57 +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>,
        Vladimir Davydov <vdavydov.dev@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, andi.kleen@...el.com,
        tim.c.chen@...el.com, dave.hansen@...el.com, ying.huang@...el.com,
        Roman Gushchin <guro@...com>
Subject: Re: [PATCH 1/2] mm: page_counter: relayout structure to reduce false
 sharing

On Tue 29-12-20 22:35:13, 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.

Yes, parent is write-once field so having it away from other heavy RW
fields makes sense to me.
 
> Following are some performance data with the patch, against
> v5.11-rc1, on several generations of Xeon platforms. Most of the
> results are improvements, with only one malloc case on one platform
> shows a -4.0% regression. Each category below has several subcases
> run on different platform, and only the worst and best scores are
> listed:
> 
> fio:				 +1.8% ~  +8.3%
> will-it-scale/malloc1:		 -4.0% ~  +8.9%
> will-it-scale/page_fault1:	 no change
> will-it-scale/page_fault2:	 +2.4% ~  +20.2%

What is the second number? Std?

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