linux-kernel - Re: [PATCH] mm/swap: Use page flags to determine LRU list in __activate

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Date:   Tue, 31 Oct 2017 13:45:39 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michal Hocko <mhocko@...nel.org>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org, shli@...nel.org
Subject: Re: [PATCH] mm/swap: Use page flags to determine LRU list in
 __activate_page()

On 10/19/2017 05:33 PM, Michal Hocko wrote:
> On Thu 19-10-17 20:26:57, Anshuman Khandual wrote:
>> Its already assumed that the PageActive flag is clear on the input
>> page, hence page_lru(page) will pick the base LRU for the page. In
>> the same way page_lru(page) will pick active base LRU, once the
>> flag PageActive is set on the page. This change of LRU list should
>> happen implicitly through the page flags instead of being hard
>> coded.
> 
> The patch description tells what but it doesn't explain _why_? Does the
> resulting code is better, more optimized or is this a pure readability
> thing?
> 
> All I can see is that page_lru is more complex and a large part of it
> can be optimized away which has been done manually here. I suspect the
> compiler can deduce the same thing.

We shouldn't overestimate the compiler (or the objective conditions it
has) for optimizing stuff away:

After applying the patch:

./scripts/bloat-o-meter swap_before.o mm/swap.o
add/remove: 0/0 grow/shrink: 1/0 up/down: 160/0 (160)
function                                     old     new   delta
__activate_page                              708     868    +160
Total: Before=13538, After=13698, chg +1.18%

I don't think we want that, it's not exactly a cold code...