linux-kernel - Re: [PATCH 2/3] mm/util.c: make vm_memory

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Message-ID: <20200515072307.GG29153@dhcp22.suse.cz>
Date:   Fri, 15 May 2020 09:23:07 +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>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Iurii Zaikin <yzaikin@...gle.com>, 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
Subject: Re: [PATCH 2/3] mm/util.c: make vm_memory_committed() more accurate

On Fri 08-05-20 15:25:16, Feng Tang wrote:
> percpu_counter_sum_positive() will provide more accurate info.

Why do we need that?

> 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 two users: /proc/meminfo and HV ballon
> driver's status trace per second.

OK, this explains that the additional overhead is not terrible but there
is no actual information on why the imprecision is a problem.

> Signed-off-by: Feng Tang <feng.tang@...el.com>
> ---
>  mm/util.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/util.c b/mm/util.c
> index 988d11e..3de78e9 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -774,7 +774,7 @@ 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