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Message-ID: <CAJDx_rjyLXxFxCG3QENN23+Xcao=_jeLTDZho0xrLm5i=Sc9GQ@mail.gmail.com>
Date: Wed, 21 May 2025 09:51:58 -0700
From: Juan Yescas <jyescas@...gle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
"Liam R. Howlett" <Liam.Howlett@...cle.com>, Vlastimil Babka <vbabka@...e.cz>,
Mike Rapoport <rppt@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
Zi Yan <ziy@...dia.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
tjmercier@...gle.com, isaacmanjarres@...gle.com, kaleshsingh@...gle.com,
masahiroy@...nel.org, Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH v6] mm: Add CONFIG_PAGE_BLOCK_ORDER to select page block order
On Tue, May 20, 2025 at 11:47 PM David Hildenbrand <david@...hat.com> wrote:
>
> On 21.05.25 00:59, Juan Yescas wrote:
> > Problem: On large page size configurations (16KiB, 64KiB), the CMA
> > alignment requirement (CMA_MIN_ALIGNMENT_BYTES) increases considerably,
> > and this causes the CMA reservations to be larger than necessary.
> > This means that system will have less available MIGRATE_UNMOVABLE and
> > MIGRATE_RECLAIMABLE page blocks since MIGRATE_CMA can't fallback to them.
> >
> > The CMA_MIN_ALIGNMENT_BYTES increases because it depends on
> > MAX_PAGE_ORDER which depends on ARCH_FORCE_MAX_ORDER. The value of
> > ARCH_FORCE_MAX_ORDER increases on 16k and 64k kernels.
> >
> > For example, in ARM, the CMA alignment requirement when:
> >
> > - CONFIG_ARCH_FORCE_MAX_ORDER default value is used
> > - CONFIG_TRANSPARENT_HUGEPAGE is set:
> >
> > PAGE_SIZE | MAX_PAGE_ORDER | pageblock_order | CMA_MIN_ALIGNMENT_BYTES
> > -----------------------------------------------------------------------
> > 4KiB | 10 | 10 | 4KiB * (2 ^ 10) = 4MiB
>
> Why is pageblock_nr_pages 10 in that case?
>
> #define pageblock_order MIN_T(unsigned int, HPAGE_PMD_ORDER, MAX_PAGE_ORDER)
>
> So it should be 2 MiB (order-9)?
>
That is right. I will update the description to set it to 2 MiB.
> > 16Kib | 11 | 11 | 16KiB * (2 ^ 11) = 32MiB
> > 64KiB | 13 | 13 | 64KiB * (2 ^ 13) = 512MiB
> >
> > There are some extreme cases for the CMA alignment requirement when:
> >
> > - CONFIG_ARCH_FORCE_MAX_ORDER maximum value is set
> > - CONFIG_TRANSPARENT_HUGEPAGE is NOT set:
> > - CONFIG_HUGETLB_PAGE is NOT set
>
> I think we should just always group at HPAGE_PMD_ORDER also in this case. But that's
> a different thing to sort out :)
>
> >
> > PAGE_SIZE | MAX_PAGE_ORDER | pageblock_order | CMA_MIN_ALIGNMENT_BYTES
> > ------------------------------------------------------------------------
> > 4KiB | 15 | 15 | 4KiB * (2 ^ 15) = 128MiB
> > 16Kib | 13 | 13 | 16KiB * (2 ^ 13) = 128MiB
> > 64KiB | 13 | 13 | 64KiB * (2 ^ 13) = 512MiB
> >
> > This affects the CMA reservations for the drivers. If a driver in a
> > 4KiB kernel needs 4MiB of CMA memory, in a 16KiB kernel, the minimal
> > reservation has to be 32MiB due to the alignment requirements:
> >
> > reserved-memory {
> > ...
> > cma_test_reserve: cma_test_reserve {
> > compatible = "shared-dma-pool";
> > size = <0x0 0x400000>; /* 4 MiB */
> > ...
> > };
> > };
> >
> > reserved-memory {
> > ...
> > cma_test_reserve: cma_test_reserve {
> > compatible = "shared-dma-pool";
> > size = <0x0 0x2000000>; /* 32 MiB */
> > ...
> > };
> > };
> >
> > Solution: Add a new config CONFIG_PAGE_BLOCK_ORDER that
> > allows to set the page block order in all the architectures.
> > The maximum page block order will be given by
> > ARCH_FORCE_MAX_ORDER.
> >
> > By default, CONFIG_PAGE_BLOCK_ORDER will have the same
> > value that ARCH_FORCE_MAX_ORDER. This will make sure that
> > current kernel configurations won't be affected by this
> > change. It is a opt-in change.
> >
> > This patch will allow to have the same CMA alignment
> > requirements for large page sizes (16KiB, 64KiB) as that
> > in 4kb kernels by setting a lower pageblock_order.
> >
> > Tests:
> >
> > - Verified that HugeTLB pages work when pageblock_order is 1, 7, 10
> > on 4k and 16k kernels.
> >
> > - Verified that Transparent Huge Pages work when pageblock_order
> > is 1, 7, 10 on 4k and 16k kernels.
> >
> > - Verified that dma-buf heaps allocations work when pageblock_order
> > is 1, 7, 10 on 4k and 16k kernels.
> >
> > Benchmarks:
> >
> > The benchmarks compare 16kb kernels with pageblock_order 10 and 7. The
> > reason for the pageblock_order 7 is because this value makes the min
> > CMA alignment requirement the same as that in 4kb kernels (2MB).
> >
> > - Perform 100K dma-buf heaps (/dev/dma_heap/system) allocations of
> > SZ_8M, SZ_4M, SZ_2M, SZ_1M, SZ_64, SZ_8, SZ_4. Use simpleperf
> > (https://developer.android.com/ndk/guides/simpleperf) to measure
> > the # of instructions and page-faults on 16k kernels.
> > The benchmark was executed 10 times. The averages are below:
> >
> > # instructions | #page-faults
> > order 10 | order 7 | order 10 | order 7
> > --------------------------------------------------------
> > 13,891,765,770 | 11,425,777,314 | 220 | 217
> > 14,456,293,487 | 12,660,819,302 | 224 | 219
> > 13,924,261,018 | 13,243,970,736 | 217 | 221
> > 13,910,886,504 | 13,845,519,630 | 217 | 221
> > 14,388,071,190 | 13,498,583,098 | 223 | 224
> > 13,656,442,167 | 12,915,831,681 | 216 | 218
> > 13,300,268,343 | 12,930,484,776 | 222 | 218
> > 13,625,470,223 | 14,234,092,777 | 219 | 218
> > 13,508,964,965 | 13,432,689,094 | 225 | 219
> > 13,368,950,667 | 13,683,587,37 | 219 | 225
> > -------------------------------------------------------------------
> > 13,803,137,433 | 13,131,974,268 | 220 | 220 Averages
> >
> > There were 4.85% #instructions when order was 7, in comparison
> > with order 10.
> >
> > 13,803,137,433 - 13,131,974,268 = -671,163,166 (-4.86%)
> >
> > The number of page faults in order 7 and 10 were the same.
> >
> > These results didn't show any significant regression when the
> > pageblock_order is set to 7 on 16kb kernels.
> >
> > - Run speedometer 3.1 (https://browserbench.org/Speedometer3.1/) 5 times
> > on the 16k kernels with pageblock_order 7 and 10.
> >
> > order 10 | order 7 | order 7 - order 10 | (order 7 - order 10) %
> > -------------------------------------------------------------------
> > 15.8 | 16.4 | 0.6 | 3.80%
> > 16.4 | 16.2 | -0.2 | -1.22%
> > 16.6 | 16.3 | -0.3 | -1.81%
> > 16.8 | 16.3 | -0.5 | -2.98%
> > 16.6 | 16.8 | 0.2 | 1.20%
> > -------------------------------------------------------------------
> > 16.44 16.4 -0.04 -0.24% Averages
> >
> > The results didn't show any significant regression when the
> > pageblock_order is set to 7 on 16kb kernels.
> >
>
> Sorry for the late reply. I think using a bootime option might have saved us
> some of the headake. :)
No worries.
The bootime option sounds good, however, there are these tradeoffs:
- bootloader needs to be updated to find out the kernel page size and calculate
the pageblock_order to pass to the kernel.
- if the pageblock_order changes, it is likely that some CMA
reservations might need
to be updated, so the DTS needs to be compiled.
> [...]
>
> > +/* Defines the order for the number of pages that have a migrate type. */
> > +#ifndef CONFIG_PAGE_BLOCK_ORDER
> > +#define PAGE_BLOCK_ORDER MAX_PAGE_ORDER
> > +#else
> > +#define PAGE_BLOCK_ORDER CONFIG_PAGE_BLOCK_ORDER
> > +#endif /* CONFIG_PAGE_BLOCK_ORDER */
> > +
> > +/*
> > + * The MAX_PAGE_ORDER, which defines the max order of pages to be allocated
> > + * by the buddy allocator, has to be larger or equal to the PAGE_BLOCK_ORDER,
> > + * which defines the order for the number of pages that can have a migrate type
> > + */
> > +#if (PAGE_BLOCK_ORDER > MAX_PAGE_ORDER)
> > +#error MAX_PAGE_ORDER must be >= PAGE_BLOCK_ORDER
> > +#endif
> > +> /*
> > * PAGE_ALLOC_COSTLY_ORDER is the order at which allocations are deemed
> > * costly to service. That is between allocation orders which should
> > diff --git a/include/linux/pageblock-flags.h b/include/linux/pageblock-flags.h
> > index fc6b9c87cb0a..e73a4292ef02 100644
> > --- a/include/linux/pageblock-flags.h
> > +++ b/include/linux/pageblock-flags.h
> > @@ -41,18 +41,18 @@ extern unsigned int pageblock_order;
> > * Huge pages are a constant size, but don't exceed the maximum allocation
> > * granularity.
> > */
>
> How is CONFIG_HUGETLB_PAGE_SIZE_VARIABLE handled?
That is a powepc configuration, and the pageorder_order variable is
initialized in:
mm/mm_init.c
#ifdef CONFIG_HUGETLB_PAGE_SIZE_VARIABLE
/* Initialise the number of pages represented by NR_PAGEBLOCK_BITS */
void __init set_pageblock_order(void)
{
unsigned int order = MAX_PAGE_ORDER;
/* Check that pageblock_nr_pages has not already been setup */
if (pageblock_order)
return;
/* Don't let pageblocks exceed the maximum allocation granularity. */
if (HPAGE_SHIFT > PAGE_SHIFT && HUGETLB_PAGE_ORDER < order)
order = HUGETLB_PAGE_ORDER;
/*
* Assume the largest contiguous order of interest is a huge page.
* This value may be variable depending on boot parameters on powerpc.
*/
pageblock_order = order;
}
Should this line be updated?
https://elixir.bootlin.com/linux/v6.15-rc7/source/mm/mm_init.c#L1513
unsigned int order = MAX_PAGE_ORDER;
> > -#define pageblock_order MIN_T(unsigned int, HUGETLB_PAGE_ORDER, MAX_PAGE_ORDER)
> > +#define pageblock_order MIN_T(unsigned int, HUGETLB_PAGE_ORDER, PAGE_BLOCK_ORDER)
> >
> > #endif /* CONFIG_HUGETLB_PAGE_SIZE_VARIABLE */
> >
> > #elif defined(CONFIG_TRANSPARENT_HUGEPAGE)
> >
> > -#define pageblock_order MIN_T(unsigned int, HPAGE_PMD_ORDER, MAX_PAGE_ORDER)
> > +#define pageblock_order MIN_T(unsigned int, HPAGE_PMD_ORDER, PAGE_BLOCK_ORDER)
>
> Wait, why are we using the MIN_T in that case? If someone requests 4 MiB, why would we reduce
> it to 2 MiB even though MAX_PAGE_ORDER allows for it?
>
I don't have the context for that change. I think Vlastimil might know
why it is needed:
That change was introduced in this patch:
https://lore.kernel.org/all/20240426040258.AD47FC113CD@smtp.kernel.org/
Thanks
Juan
>
> Maybe we really have to clean all that up first :/
>
> --
> Cheers,
>
> David / dhildenb
>
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