linux-kernel - Re: [PATCH -mm -v2 2/6] mm, swap: Add swap readahead hit statistics

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Date:   Tue, 11 Jul 2017 11:25:36 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Huang, Ying" <ying.huang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Minchan Kim <minchan@...nel.org>,
        Rik van Riel <riel@...hat.com>, Shaohua Li <shli@...nel.org>,
        Hugh Dickins <hughd@...gle.com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Tim Chen <tim.c.chen@...el.com>
Subject: Re: [PATCH -mm -v2 2/6] mm, swap: Add swap readahead hit statistics

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?