linux-kernel - Re: [PATCH v4 2/4] mm/util.c: make vm_memory

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Date:   Wed, 3 Jun 2020 15:35:28 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Feng Tang <feng.tang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Kees Cook <keescook@...omium.org>, Qian Cai <cai@....pw>,
        andi.kleen@...el.com, tim.c.chen@...el.com, dave.hansen@...el.com,
        ying.huang@...el.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>
Subject: Re: [PATCH v4 2/4] mm/util.c: make vm_memory_committed() more
 accurate

On Fri 29-05-20 09:06:08, Feng Tang wrote:
> percpu_counter_sum_positive() will provide more accurate info.
> 
> As with percpu_counter_read_positive(), in worst case the deviation
> could be 'batch * nr_cpus', which is totalram_pages/256 for now,
> and will be more when the batch gets enlarged.
> 
> Its time cost is about 800 nanoseconds on a 2C/4T platform and
> 2~3 microseconds on a 2S/36C/72T server in normal case, and in
> worst case where vm_committed_as's spinlock is under severe
> contention, it costs 30~40 microseconds for the 2S/36C/72T sever,
> which should be fine for its only two users: /proc/meminfo and
> HyperV balloon driver's status trace per second.
> 
> Signed-off-by: Feng Tang <feng.tang@...el.com>

I cannot speak for HyperV part. Cc maintainers but this shouldn't be a
problem for meminfo.

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

> ---
>  mm/util.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/util.c b/mm/util.c
> index 9b3be03..3c7a08c 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -790,7 +790,7 @@ struct percpu_counter vm_committed_as ____cacheline_aligned_in_smp;
>   */
>  unsigned long vm_memory_committed(void)
>  {
> -	return percpu_counter_read_positive(&vm_committed_as);
> +	return percpu_counter_sum_positive(&vm_committed_as);
>  }
>  EXPORT_SYMBOL_GPL(vm_memory_committed);
>  
> -- 
> 2.7.4
> 

-- 
Michal Hocko
SUSE Labs