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

[Roman Gushchin]

On Tue, Dec 29, 2020 at 10:35:13PM +0800, 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, 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%
> 
> [1].https://lore.kernel.org/lkml/20201102091543.GM31092@shao2-debian/
> Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
> Cc: Roman Gushchin <guro@xxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> ---
>  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;
>  };

LGTM!

Reviewed-by: Roman Gushchin <guro@xxxxxx>

I wonder if we have the same problem with min/low/high/max?
Maybe try to group all mostly-read-only fields (min, low, high,
max and parent) and separate them with some padding?

Thank you!