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Date:   Wed, 3 Jun 2020 15:35:28 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Feng Tang <feng.tang@intel.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Matthew Wilcox <willy@infradead.org>,
        Mel Gorman <mgorman@suse.de>,
        Kees Cook <keescook@chromium.org>, Qian Cai <cai@lca.pw>,
        andi.kleen@intel.com, tim.c.chen@intel.com, dave.hansen@intel.com,
        ying.huang@intel.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>
Subject: Re: [PATCH v4 2/4] mm/util.c: make vm_memory_committed() more
 accurate
Message-ID: <20200603133528.GJ7533@dhcp22.suse.cz>
References: <1590714370-67182-1-git-send-email-feng.tang@intel.com>
 <1590714370-67182-3-git-send-email-feng.tang@intel.com>
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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@intel.com>

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

Acked-by: Michal Hocko <mhocko@suse.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