Re: [PATCH] mm: increase totalram_pages on freeing to buddy system

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

On Mon, Jun 03, 2024 at 10:43:51PM +0200, David Hildenbrand wrote:
>On 03.06.24 22:01, Wei Yang wrote:
>> On Mon, Jun 03, 2024 at 10:55:10AM +0200, David Hildenbrand wrote:
>> > On 02.06.24 02:58, Wei Yang wrote:
>> > > On Sat, Jun 01, 2024 at 06:15:33PM +0200, David Hildenbrand wrote:
>> > > > On 01.06.24 17:32, David Hildenbrand wrote:
>> > > > > On 01.06.24 15:34, Wei Yang wrote:
>> > > > > > Total memory represents pages managed by buddy system.
>> > > > > 
>> > > > > No, that's managed pages.
>> > > > > 
>> > > > > > After the
>> > > > > > introduction of DEFERRED_STRUCT_PAGE_INIT, it may count the pages before
>> > > > > > being managed.
>> > > > > > 
>> > > > > 
>> > > > > I recall one reason that is done, so other subsystem know the total
>> > > > > memory size even before deferred init is done.
>> > > > > 
>> > > > > > free_low_memory_core_early() returns number of pages for all free pages,
>> > > > > > even at this moment only early initialized pages are freed to buddy
>> > > > > > system. This means the total memory at this moment is not correct.
>> > > > > > 
>> > > > > > Let's increase it when pages are freed to buddy system.
>> > > > > 
>> > > > > I'm missing the "why", and the very first sentence of this patch is wrong.
>> > > > 
>> > > > Correction: your statement was correct :) That's why
>> > > > adjust_managed_page_count() adjusts that as well.
>> > > > 
>> > > > __free_pages_core() only adjusts managed page count, because it assumes
>> > > > totalram has already been adjusted early during boot.
>> > > > 
>> > > > The reason we have this split for now, I think, is because of subsystems that
>> > > > call totalram_pages() during init.
>> > > > 
>> > > > So the "why" question remains, because this change has the potential to break
>> > > > other stuff.
>> > > > 
>> > > 
>> > > Thanks, I didn't notice this.
>> > 
>> > I think having your cleanup would be very nice, as I have patches in the
>> > works that would benefit from being able to move the totalram update from
>> > memory hotplug code to __free_pages_core().
>> > 
>> 
>> I got the same feeling.
>> 
>> > We'd have to make sure that no code relies on totalram being sane/fixed
>> > during boot for the initial memory. I think right now we might have such
>> > code.
>> > 
>> 
>> One concern is totalram would change when hotplug is enabled. That sounds
>> those codes should do some re-calculation after totalram changes?
>
>We don't have such code in place -- there were discussions regarding that
>recently.
>
>It would be reasonable to take a look at all totalram_pages() users and
>determine if they could be affected by deferring updating it.
>
>At least page_alloc_init_late()->deferred_init_memmap() happens before
>do_basic_setup()->do_initcalls(), which is good.
>

But deferred_init_memmap() will spawn threads to do the work. I am afraid
do_initcalls() won't wait for the completion of defer_init? Do I miss
something?

>So maybe it's not a big concern and this separate totalram pages accounting
>is much rather some legacy leftover.
>
>> 
>> > Further, we currently require only a single atomic RMW instruction to adjust
>> > totalram during boot, moving it to __free_pages_core() would imply more
>> > atomics: but usually only one per MAX_ORDER page, so I doubt this would make
>> > a big difference.
>> > 
>> 
>> I took a rough calculation on this.One MAX_ORDER page accounts for 2MB, and
>> with defer_init only low zone's memory is initialized during boot. Per my
>> understanding, low zone's memory is 4GB for x86. So the extra calculation is
>> 4GB / 2MB = 2K.
>
>Well, for all deferred-initialized memory you would now also require these --
>or if deferred-init would be disabled. Sounds like an interesting measurement
>if that would be measurable at all.
>
>-- 
>Cheers,
>
>David / dhildenb

-- 
Wei Yang
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