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Date:   Mon, 22 Aug 2022 12:23:11 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Shakeel Butt <shakeelb@google.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Muchun Song <songmuchun@bytedance.com>,
        Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>,
        Eric Dumazet <edumazet@google.com>,
        Soheil Hassas Yeganeh <soheil@google.com>,
        Feng Tang <feng.tang@intel.com>,
        Oliver Sang <oliver.sang@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>, lkp@lists.01.org,
        cgroups@vger.kernel.org, linux-mm@kvack.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/3] mm: page_counter: rearrange struct page_counter
 fields
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 <20220822001737.4120417-3-shakeelb@google.com>
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On Mon 22-08-22 00:17:36, Shakeel Butt 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.

Again the workload description is not particularly useful. I guess the
only important aspect is the netserver part below and the number of CPUs
because min and low setup doesn't have much to do with this, right? At
least that is my reading of the memory.high mentioned above.

>  $ 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%.
> 
> 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@google.com>
> Reported-by: kernel test robot <oliver.sang@intel.com>
> ---
>  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_);

Why don't you simply require alignment for the structure?

Other than that, looks good to me and it makes sense.

-- 
Michal Hocko
SUSE Labs