Re: [PATCH] mm, page_alloc: skip zone who has no managed_pages in calculate_totalreserve_pages()

[Wei Yang <richard.weiyang@gmail.com>]

On Tue, Nov 13, 2018 at 10:07:58AM +0100, Michal Hocko wrote:
>On Tue 13-11-18 08:16:44, Wei Yang wrote:
>> On Tue, Nov 13, 2018 at 09:08:34AM +0100, Michal Hocko wrote:
>> >On Tue 13-11-18 01:39:42, Wei Yang wrote:
>> >> On Mon, Nov 12, 2018 at 03:40:20PM +0100, Michal Hocko wrote:
>> >> >On Mon 12-11-18 14:26:41, Wei Yang wrote:
>> >> >> On Mon, Nov 12, 2018 at 09:09:26AM +0100, Michal Hocko wrote:
>> >> >> >On Mon 12-11-18 15:14:04, Wei Yang wrote:
>> >> >> >> Zone with no managed_pages doesn't contribute totalreserv_pages. And the
>> >> >> >> more nodes we have, the more empty zones there are.
>> >> >> >> 
>> >> >> >> This patch skip the zones to save some cycles.
>> >> >> >
>> >> >> >What is the motivation for the patch? Does it really cause any
>> >> >> >measurable difference in performance?
>> >> >> >
>> >> >> 
>> >> >> The motivation here is to reduce some unnecessary work.
>> >> >
>> >> >I have guessed so even though the changelog was quite modest on the
>> >> >motivation.
>> >> >
>> >> >> Based on my understanding, almost every node has empty zones, since
>> >> >> zones within a node are ordered in monotonic increasing memory address.
>> >> >
>> >> >Yes, this is likely the case. Btw. a check for populated_zone or
>> >> >for_each_populated_zone would suite much better.
>> >> >
>> >> 
>> >> Hmm... maybe not exact.
>> >> 
>> >>     populated_zone checks zone->present_pages
>> >>     managed_zone checks zone->managed_pages
>> >> 
>> >> As the comment of managed_zone says, this one records the pages managed
>> >> by buddy system. And when we look at the usage of totalreserve_pages, it
>> >> is only used in page allocation. And finally, *max* is checked with
>> >> managed_pages instead of present_pages.
>> >> 
>> >> Because of this, managed_zone is more accurate at this place. Is my
>> >> understanding correct?
>> >
>> >OK, fair enough. There is a certain discrepancy here. You are right that
>> >we do not care about pages out of the page allocator scope (e.g. early
>> >bootmem allocations, struct pages) but this is likely what other callers
>> >of populated_zone are looking for as well. It seems that managed pages
>> >counter which only came in later was not considered in other places.
>> >
>> >That being said this asks for a cleanup of some sort. And I think such a
>> >cleanup wold be appreciated much more than an optimization of an unknown
>> >effect and wonder why this check is used here and not at other places.
>> 
>> You are right. There are three pages(spanned, managed, present) in a
>> zone, which is a little confusing.
>> 
>> So you are willing to get rid of present_pages, if I am right?
>
>No, I believe we want all three of them. But reviewing
>for_each_populated_zone users and explicit checks for present/managed
>pages and unify them would be a step forward both a more optimal code
>and more maintainable code. I haven't checked but
>for_each_populated_zone would seem like a proper user for managed page
>counter. But that really requires to review all current users.

Got your point. Let me take a look to see if I could make a cleanup.

>
>-- 
>Michal Hocko
>SUSE Labs

-- 
Wei Yang
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