[PATCH v2 3/5] mm: Default implementation of arch_wants_pte_order()

[Yu Zhao]

On Tue, Jul 4, 2023 at 6:36 AM Ryan Roberts <ryan.roberts at arm.com> wrote:
>
> On 04/07/2023 04:59, Yu Zhao wrote:
> > On Mon, Jul 3, 2023 at 9:02 PM Yu Zhao <yuzhao at google.com> wrote:
> >>
> >> On Mon, Jul 3, 2023 at 8:23 PM Yin, Fengwei <fengwei.yin at intel.com> wrote:
> >>>
> >>>
> >>>
> >>> On 7/3/2023 9:53 PM, Ryan Roberts wrote:
> >>>> arch_wants_pte_order() can be overridden by the arch to return the
> >>>> preferred folio order for pte-mapped memory. This is useful as some
> >>>> architectures (e.g. arm64) can coalesce TLB entries when the physical
> >>>> memory is suitably contiguous.
> >>>>
> >>>> The first user for this hint will be FLEXIBLE_THP, which aims to
> >>>> allocate large folios for anonymous memory to reduce page faults and
> >>>> other per-page operation costs.
> >>>>
> >>>> Here we add the default implementation of the function, used when the
> >>>> architecture does not define it, which returns the order corresponding
> >>>> to 64K.
> >>>>
> >>>> Signed-off-by: Ryan Roberts <ryan.roberts at arm.com>
> >>>> ---
> >>>>  include/linux/pgtable.h | 13 +++++++++++++
> >>>>  1 file changed, 13 insertions(+)
> >>>>
> >>>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> >>>> index a661a17173fa..f7e38598f20b 100644
> >>>> --- a/include/linux/pgtable.h
> >>>> +++ b/include/linux/pgtable.h
> >>>> @@ -13,6 +13,7 @@
> >>>>  #include <linux/errno.h>
> >>>>  #include <asm-generic/pgtable_uffd.h>
> >>>>  #include <linux/page_table_check.h>
> >>>> +#include <linux/sizes.h>
> >>>>
> >>>>  #if 5 - defined(__PAGETABLE_P4D_FOLDED) - defined(__PAGETABLE_PUD_FOLDED) - \
> >>>>       defined(__PAGETABLE_PMD_FOLDED) != CONFIG_PGTABLE_LEVELS
> >>>> @@ -336,6 +337,18 @@ static inline bool arch_has_hw_pte_young(void)
> >>>>  }
> >>>>  #endif
> >>>>
> >>>> +#ifndef arch_wants_pte_order
> >>>> +/*
> >>>> + * Returns preferred folio order for pte-mapped memory. Must be in range [0,
> >>>> + * PMD_SHIFT-PAGE_SHIFT) and must not be order-1 since THP requires large folios
> >>>> + * to be at least order-2.
> >>>> + */
> >>>> +static inline int arch_wants_pte_order(struct vm_area_struct *vma)
> >>>> +{
> >>>> +     return ilog2(SZ_64K >> PAGE_SHIFT);
> >>> Default value which is not related with any silicon may be: PAGE_ALLOC_COSTLY_ORDER?
> >>>
> >>> Also, current pcp list support cache page with order 0...PAGE_ALLOC_COSTLY_ORDER, 9.
> >>> If the pcp could cover the page, the pressure to zone lock will be reduced by pcp.
> >>
> >> The value of PAGE_ALLOC_COSTLY_ORDER is reasonable but again it's a
> >> s/w policy not a h/w preference. Besides, I don't think we can include
> >> mmzone.h in pgtable.h.
> >
> > I think we can make a compromise:
> > 1. change the default implementation of arch_has_hw_pte_young() to return 0, and
> > 2. in memory.c, we can try PAGE_ALLOC_COSTLY_ORDER for archs that
> > don't override arch_has_hw_pte_young(), or if its return value is too
> > large to fit.
> > This should also take care of the regression, right?
>
> I think you are suggesting that we use 0 as a sentinel which we then translate
> to PAGE_ALLOC_COSTLY_ORDER? I already have a max_anon_folio_order() function in
> memory.c (actually it is currently a macro defined as arch_wants_pte_order()).
>
> So it would become (I'll talk about the vma concern separately in the thread
> where you raised it):
>
> static inline int max_anon_folio_order(struct vm_area_struct *vma)
> {
>         int order = arch_wants_pte_order(vma);
>
>         return order ? order : PAGE_ALLOC_COSTLY_ORDER;
> }
>
> Correct?
>
> I don't see how it fixes the regression (assume you're talking about
> Speedometer) though? On arm64 arch_wants_pte_order() will still be returning
> order-4.

Here is what I was actually suggesting -- I think the problem was
because contpte is a bit too large for that benchmark and for the page
allocator too, unfortunately. The following allows one retry (32KB)
before fallback to order 0 when using contpte (64KB). There is no
retry for HPA (16KB) and other archs.

+       int preferred = arch_wants_pte_order(vma) ? : PAGE_ALLOC_COSTLY_ORDER;
+       int orders[] = {
+               preferred,
+               preferred > PAGE_ALLOC_COSTLY_ORDER ?
PAGE_ALLOC_COSTLY_ORDER : 0,
+               0,
+       };

I'm attaching a patch which fills in the two helpers I left empty here [1].

Would the above work for Intel, Fengwei?

(AMD wouldn't need to override arch_wants_pte_order() since PTE
coalescing on Zen is also PAGE_ALLOC_COSTLY_ORDER.)

[1] https://lore.kernel.org/linux-mm/CAOUHufaK82K8Sa35T7z3=gkm4GB0cWD3aqeZF6mYx82v7cOTeA@mail.gmail.com/2-anon_folios.patch
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