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 Help you, Help me
