Re: [PATCH] mm/swap: Use page flags to determine LRU list in __activate_page()

[Anshuman Khandual]

On 10/31/2017 06:15 PM, Vlastimil Babka wrote:
> 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...

Yeah, makes sense.