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, Kalesh Singh , Marc Zyngier , Mark Rutland , Matthias Brugger , Miroslav Benes , Oliver Upton , Will Deacon , kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [RFC PATCH v1 57/57] arm64: Enable boot-time page size selection Date: Wed, 16 Oct 2024 10:35:01 -0400 X-Mailer: MailMate (1.14r6065) Message-ID: <8B1AC3A8-514F-48C1-9F9C-A4A29E129788@nvidia.com> In-Reply-To: <401175cf-e205-4e5c-939f-60b7ab948355@arm.com> References: <20241014105514.3206191-1-ryan.roberts@arm.com> <20241014105912.3207374-1-ryan.roberts@arm.com> <20241014105912.3207374-57-ryan.roberts@arm.com> <6C0A53C0-C332-46B3-A84A-3232E9D71003@nvidia.com> <401175cf-e205-4e5c-939f-60b7ab948355@arm.com> Content-Type: multipart/signed; boundary="=_MailMate_3B303FDF-2AEE-4550-9D2F-014729012A86_="; micalg=pgp-sha512; protocol="application/pgp-signature" X-ClientProxiedBy: BL1PR13CA0392.namprd13.prod.outlook.com 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Roberts wrote: >>>> >>>>> Introduce a new Kconfig, ARM64_BOOT_TIME_PAGE_SIZE, which can be >>>>> selected instead of a page size. When selected, the resulting kernel's >>>>> page size can be configured at boot via the command line. >>>>> >>>>> For now, boot-time page size kernels are limited to 48-bit VA, since >>>>> more work is required to support LPA2. Additionally MMAP_RND_BITS and >>>>> SECTION_SIZE_BITS are configured for the worst case (64K pages). Future >>>>> work could be implemented to be able to configure these at boot time for >>>>> optimial page size-specific values. >>>>> >>>>> Signed-off-by: Ryan Roberts >>>>> --- >>>> >>>> >>>> >>>>> >>>>> @@ -1588,9 +1601,10 @@ config XEN >>>>> # 4K | 27 | 12 | 15 | 10 | >>>>> # 16K | 27 | 14 | 13 | 11 | >>>>> # 64K | 29 | 16 | 13 | 13 | >>>>> +# BOOT| 29 | 16 (max) | 13 | 13 | >>>>> config ARCH_FORCE_MAX_ORDER >>>>> int >>>>> - default "13" if ARM64_64K_PAGES >>>>> + default "13" if ARM64_64K_PAGES || ARM64_BOOT_TIME_PAGE_SIZE >>>>> default "11" if ARM64_16K_PAGES >>>>> default "10" >>>>> help >>>> >>>> So boot-time page size kernel always has the highest MAX_PAGE_ORDER, which >>>> means the section size increases for 4KB and 16KB page sizes. Any downside >>>> for this? >>> >>> I guess there is some cost to the buddy when MAX_PAGE_ORDER is larger than it >>> needs to be - I expect you can explain those details much better than I can. I'm >>> just setting it to the worst case for now as it was the easiest solution for the >>> initial series. >> >> From my past experience (around 5.19), the perf impact (using vm-scalability) >> seems very small due to MAX_PAGE_ORDER increases [1] (I made MAX_PAGE_ORDER >> a boot time variable and increased it to 20 for my 1GB THP experiments). >> >> Larger MAX_PAGE_ORDER means larger section size and larger mem_block size, >> so the granularity of memory hotplug also increases. In this case: >> 1. ARM64 4KB: mem_block size increases from 4MB to 32MB, >> 2. ARM64 16KB: mem_block size increases from 32MB to 128MB, >> 3. ARM64 64KB: mem_block size keeps the same, 512MB. >> >> DavidH was concerned about large mem_block size before. He might have some >> opinion on this. >> >> >>> >>>> >>>> Is there any plan (not in this patchset) to support boot-time MAX_PAGE_ORDER >>>> to keep section size the same? >>> >>> Yes absolutely. I should have documented MAX_PAGE_ORDER in the commit log along >>> with the comments for MMAP_RND_BITS and SECTION_SIZE_BITS - that was an >>> oversight and I'll fix it in the next version. I plan to look at making all 3 >>> values boot-time configurable in future (although I have no idea at this point >>> how involved that will be). >> >> In [1], I tried to make MAX_PAGE_ORDER a boot time variable, >> but for a different purpose, allocating 1GB THP. I needed some additional >> changes in my patchset, since I assumed MAX_PAGE_ORDER can go beyond >> section size, which makes things a little bit complicated. For your case, >> I assume you are not planning to make MAX_PAGE_ORDER bigger than section >> size, then I should be able to revive my patchset with fewer changes. > > Yes correct; no need to make it bigger than section size. Thanks for the patch, > I'll certainly use it as a base when I get there or if you're interested in > doing it then even better ;-) > > But I don't think this is urgent. For now, boot-time page size is a new Kconfig > for arm64. It still supports the compile-time page size options. So having a > larger MAX_PAGE_ORDER than strictly necessary doesn't represent a regression, > just a limitation of boot-time page size config - something we can optimize later. Sure. I will revisit my boot time MAX_PAGE_ORDER patchset when this patchset settles. Glad to help. :) > >> >> In terms of SECTION_SIZE_BITS, why do you want to make it a boot time variable? >> Since it decides the minimum memory hotplug size, I assume we should keep >> it unchanged or as small as possible to make virtual machine memory usage >> efficient. > > When I say "boot-time variable" I just mean something that the arch can > configure at boot based on the selected page size. I'm not proposing to allow > the user to set it via the command line. That means we need to rid the code of > any assumptions that it is compile time constant (e.g. c preprocessor usage of > the value, etc). The same goes for MAX_PAGE_ORDER and the MMAP_RND_BITS stuff. > Got it. >> [1] https://lore.kernel.org/linux-mm/20220811231643.1012912-1-zi.yan@sent.com/ Best Regards, Yan, Zi --=_MailMate_3B303FDF-2AEE-4550-9D2F-014729012A86_= Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename=signature.asc Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iQJDBAEBCgAtFiEEh7yFAW3gwjwQ4C9anbJR82th+ooFAmcPzxUPHHppeUBudmlk aWEuY29tAAoJEJ2yUfNrYfqKDcsP/0FwZafyBDetn+/HtBMm63Vi2W+LZ0OOM0Tx wOhIy79WDEsDstv7N3ePwLUKSjR4aL3mxVxMpPHyZwjBV9lRravBXqvyevkMOmgS OlY0Lhj6hGDAx3lwp8buRImXnwhnkb+kyyaYhYi+4KnjLLvswIjZ1puAwzIvllnz pq0Loz2g9FkVLtohE4gAoQt6XBgolsuqb9nfDRL/erAfp3wkMC3RsrbigxjZ2Bdd vRvyIizup0DLFhJa1sBxNdLy6Rl+kZ6JSYNaYFSW+im3zKYKnl/Gp+bKfLqM2Chj 1mGrntGETpZawzRc/xfUfmiLSkkni7uK5wjnpXoKH9HuOIDMj4PA/Zd6lhZMtKj0 GKxElckfvGAP10jbAFQc8zHgDfLgL0IZSppzG5Zg6AND+2JZwmLxsduSrIUcXMfW EHIL6vhunUq9ScW0MDGQeN4FFHiLoy4AiydV8LbVY1RlNZvQaK2OAAzQEnYD+Y1x 5qmXTf1iw0TirU5+95awoogYnvktNzheKfLBXzuIHjpaKbVQ6sR22UZgsWkI+r2I godwyL7mzgg9KWFUUIUrpUlnF0oEa2aBaMOTqFw8JdFTYAeKcq8dKmmhhRZaQ4zk 0pWH53yW54Gm+h8zveIsAYXTz/fGwWQqAVn/Xec85e2DZTj/KfHeTd1qnyjIk+cz wQa95b/P =5CIG -----END PGP SIGNATURE----- --=_MailMate_3B303FDF-2AEE-4550-9D2F-014729012A86_=--