Re: [PATCH 2/3] mm: page_counter: rearrange struct page_counter fields

[Soheil Hassas Yeganeh]

On Sun, Aug 21, 2022 at 8:18 PM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:
>
> With memcg v2 enabled, memcg->memory.usage is a very hot member for
> the workloads doing memcg charging on multiple CPUs concurrently.
> Particularly the network intensive workloads. In addition, there is a
> false cache sharing between memory.usage and memory.high on the charge
> path. This patch moves the usage into a separate cacheline and move all
> the read most fields into separate cacheline.
>
> To evaluate the impact of this optimization, on a 72 CPUs machine, we
> ran the following workload in a three level of cgroup hierarchy with top
> level having min and low setup appropriately. More specifically
> memory.min equal to size of netperf binary and memory.low double of
> that.
>
>  $ netserver -6
>  # 36 instances of netperf with following params
>  $ netperf -6 -H ::1 -l 60 -t TCP_SENDFILE -- -m 10K
>
> Results (average throughput of netperf):
> Without (6.0-rc1)       10482.7 Mbps
> With patch              12413.7 Mbps (18.4% improvement)
>
> With the patch, the throughput improved by 18.4%.

Shakeel, for my understanding: is this on top of the gains from the
previous patch?

> One side-effect of this patch is the increase in the size of struct
> mem_cgroup. However for the performance improvement, this additional
> size is worth it. In addition there are opportunities to reduce the size
> of struct mem_cgroup like deprecation of kmem and tcpmem page counters
> and better packing.
>
> Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
> ---
>  include/linux/page_counter.h | 34 +++++++++++++++++++++++-----------
>  1 file changed, 23 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/page_counter.h b/include/linux/page_counter.h
> index 679591301994..8ce99bde645f 100644
> --- a/include/linux/page_counter.h
> +++ b/include/linux/page_counter.h
> @@ -3,15 +3,27 @@
>  #define _LINUX_PAGE_COUNTER_H
>
>  #include <linux/atomic.h>
> +#include <linux/cache.h>
>  #include <linux/kernel.h>
>  #include <asm/page.h>
>
> +#if defined(CONFIG_SMP)
> +struct pc_padding {
> +       char x[0];
> +} ____cacheline_internodealigned_in_smp;
> +#define PC_PADDING(name)       struct pc_padding name
> +#else
> +#define PC_PADDING(name)
> +#endif
> +
>  struct page_counter {
> +       /*
> +        * Make sure 'usage' does not share cacheline with any other field. The
> +        * memcg->memory.usage is a hot member of struct mem_cgroup.
> +        */
> +       PC_PADDING(_pad1_);
>         atomic_long_t usage;
> -       unsigned long min;
> -       unsigned long low;
> -       unsigned long high;
> -       unsigned long max;
> +       PC_PADDING(_pad2_);
>
>         /* effective memory.min and memory.min usage tracking */
>         unsigned long emin;
> @@ -23,16 +35,16 @@ struct page_counter {
>         atomic_long_t low_usage;
>         atomic_long_t children_low_usage;
>
> -       /* 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.
> -        */
> +       /* Keep all the read most fields in a separete cacheline. */
> +       PC_PADDING(_pad3_);
> +
> +       unsigned long min;
> +       unsigned long low;
> +       unsigned long high;
> +       unsigned long max;
>         struct page_counter *parent;
>  };
>
> --
> 2.37.1.595.g718a3a8f04-goog
>