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Subject: Re: [PATCH -mm -v2 2/6] mm, swap: Add swap readahead hit statistics
To: "Huang, Ying" <ying.huang@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>
References: <20170630014443.23983-1-ying.huang@intel.com>
 <20170630014443.23983-3-ying.huang@intel.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Minchan Kim <minchan@kernel.org>, Rik van Riel <riel@redhat.com>,
        Shaohua Li <shli@kernel.org>, Hugh Dickins <hughd@google.com>,
        Fengguang Wu <fengguang.wu@intel.com>, Tim Chen <tim.c.chen@intel.com>
From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <1152d4f5-fe8b-b46c-9d6b-3ecf69019172@intel.com>
Date: Tue, 11 Jul 2017 11:25:36 -0700
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On 06/29/2017 06:44 PM, Huang, Ying wrote:
>  
>  static atomic_t swapin_readahead_hits = ATOMIC_INIT(4);
> +static atomic_long_t swapin_readahead_hits_total = ATOMIC_INIT(0);
> +static atomic_long_t swapin_readahead_total = ATOMIC_INIT(0);
>  
>  void show_swap_cache_info(void)
>  {
> @@ -305,8 +307,10 @@ struct page * lookup_swap_cache(swp_entry_t entry)
>  
>  	if (page && likely(!PageTransCompound(page))) {
>  		INC_CACHE_INFO(find_success);
> -		if (TestClearPageReadahead(page))
> +		if (TestClearPageReadahead(page)) {
>  			atomic_inc(&swapin_readahead_hits);
> +			atomic_long_inc(&swapin_readahead_hits_total);
> +		}
>  	}

Adding global atomics that we touch in hot paths seems like poor
future-proofing.  Are we sure we want to do this and not use some of the
nice, fancy, percpu counters that we have?