Re: [PATCH v4 2/5] mm: LARGE_ANON_FOLIO for improved performance

From: Yu Zhao
Date: Tue Aug 01 2023 - 02:37:40 EST

On Fri, Jul 28, 2023 at 4:13 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>
> On 27/07/2023 05:31, Yu Zhao wrote:
> > On Wed, Jul 26, 2023 at 10:41 AM Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
> >>
> >> On Wed, Jul 26, 2023 at 3:52 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
> >>>
> >>> Introduce LARGE_ANON_FOLIO feature, which allows anonymous memory to be
> >>> allocated in large folios of a determined order. All pages of the large
> >>> folio are pte-mapped during the same page fault, significantly reducing
> >>> the number of page faults. The number of per-page operations (e.g. ref
> >>> counting, rmap management lru list management) are also significantly
> >>> reduced since those ops now become per-folio.
> >>>
> >>> The new behaviour is hidden behind the new LARGE_ANON_FOLIO Kconfig,
> >>> which defaults to disabled for now; The long term aim is for this to
> >>> defaut to enabled, but there are some risks around internal
> >>> fragmentation that need to be better understood first.
> >>>
> >>> When enabled, the folio order is determined as such: For a vma, process
> >>> or system that has explicitly disabled THP, we continue to allocate
> >>> order-0. THP is most likely disabled to avoid any possible internal
> >>> fragmentation so we honour that request.
> >>>
> >>> Otherwise, the return value of arch_wants_pte_order() is used. For vmas
> >>> that have not explicitly opted-in to use transparent hugepages (e.g.
> >>> where thp=madvise and the vma does not have MADV_HUGEPAGE), then
> >>> arch_wants_pte_order() is limited to 64K (or PAGE_SIZE, whichever is
> >>> bigger). This allows for a performance boost without requiring any
> >>> explicit opt-in from the workload while limitting internal
> >>> fragmentation.
> >>>
> >>> If the preferred order can't be used (e.g. because the folio would
> >>> breach the bounds of the vma, or because ptes in the region are already
> >>> mapped) then we fall back to a suitable lower order; first
> >>> PAGE_ALLOC_COSTLY_ORDER, then order-0.
> >>>
> >>> arch_wants_pte_order() can be overridden by the architecture if desired.
> >>> Some architectures (e.g. arm64) can coalsece TLB entries if a contiguous
> >>> set of ptes map physically contigious, naturally aligned memory, so this
> >>> mechanism allows the architecture to optimize as required.
> >>>
> >>> Here we add the default implementation of arch_wants_pte_order(), used
> >>> when the architecture does not define it, which returns -1, implying
> >>> that the HW has no preference. In this case, mm will choose it's own
> >>> default order.
> >>>
> >>> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> >>> ---
> >>> include/linux/pgtable.h | 13 ++++
> >>> mm/Kconfig | 10 +++
> >>> mm/memory.c | 166 ++++++++++++++++++++++++++++++++++++----
> >>> 3 files changed, 172 insertions(+), 17 deletions(-)
> >>>
> >>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> >>> index 5063b482e34f..2a1d83775837 100644
> >>> --- a/include/linux/pgtable.h
> >>> +++ b/include/linux/pgtable.h
> >>> @@ -313,6 +313,19 @@ 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. Negative value implies that the HW has no preference
> >>> + * and mm will choose it's own default order.
> >>> + */
> >>> +static inline int arch_wants_pte_order(void)
> >>> +{
> >>> + return -1;
> >>> +}
> >>> +#endif
> >>> +
> >>> #ifndef __HAVE_ARCH_PTEP_GET_AND_CLEAR
> >>> static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
> >>> unsigned long address,
> >>> diff --git a/mm/Kconfig b/mm/Kconfig
> >>> index 09130434e30d..fa61ea160447 100644
> >>> --- a/mm/Kconfig
> >>> +++ b/mm/Kconfig
> >>> @@ -1238,4 +1238,14 @@ config LOCK_MM_AND_FIND_VMA
> >>>
> >>> source "mm/damon/Kconfig"
> >>>
> >>> +config LARGE_ANON_FOLIO
> >>> + bool "Allocate large folios for anonymous memory"
> >>> + depends on TRANSPARENT_HUGEPAGE
> >>> + default n
> >>> + help
> >>> + Use large (bigger than order-0) folios to back anonymous memory where
> >>> + possible, even for pte-mapped memory. This reduces the number of page
> >>> + faults, as well as other per-page overheads to improve performance for
> >>> + many workloads.
> >>> +
> >>> endmenu
> >>> diff --git a/mm/memory.c b/mm/memory.c
> >>> index 01f39e8144ef..64c3f242c49a 100644
> >>> --- a/mm/memory.c
> >>> +++ b/mm/memory.c
> >>> @@ -4050,6 +4050,127 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >>> return ret;
> >>> }
> >>>
> >>> +static bool vmf_pte_range_changed(struct vm_fault *vmf, int nr_pages)
> >>> +{
> >>> + int i;
> >>> +
> >>> + if (nr_pages == 1)
> >>> + return vmf_pte_changed(vmf);
> >>> +
> >>> + for (i = 0; i < nr_pages; i++) {
> >>> + if (!pte_none(ptep_get_lockless(vmf->pte + i)))
> >>> + return true;
> >>> + }
> >>> +
> >>> + return false;
> >>> +}
> >>> +
> >>> +#ifdef CONFIG_LARGE_ANON_FOLIO
> >>> +#define ANON_FOLIO_MAX_ORDER_UNHINTED \
> >>> + (ilog2(max_t(unsigned long, SZ_64K, PAGE_SIZE)) - PAGE_SHIFT)
> >>> +
> >>> +static int anon_folio_order(struct vm_area_struct *vma)
> >>> +{
> >>> + int order;
> >>> +
> >>> + /*
> >>> + * If THP is explicitly disabled for either the vma, the process or the
> >>> + * system, then this is very likely intended to limit internal
> >>> + * fragmentation; in this case, don't attempt to allocate a large
> >>> + * anonymous folio.
> >>> + *
> >>> + * Else, if the vma is eligible for thp, allocate a large folio of the
> >>> + * size preferred by the arch. Or if the arch requested a very small
> >>> + * size or didn't request a size, then use PAGE_ALLOC_COSTLY_ORDER,
> >>> + * which still meets the arch's requirements but means we still take
> >>> + * advantage of SW optimizations (e.g. fewer page faults).
> >>> + *
> >>> + * Finally if thp is enabled but the vma isn't eligible, take the
> >>> + * arch-preferred size and limit it to ANON_FOLIO_MAX_ORDER_UNHINTED.
> >>> + * This ensures workloads that have not explicitly opted-in take benefit
> >>> + * while capping the potential for internal fragmentation.
> >>> + */
> >>
> >> What empirical evidence is SZ_64K based on?
> >> What workloads would benefit from it?
> >> How much would they benefit from it?
> >> Would they benefit more or less from different values?
> >> How much internal fragmentation would it cause?
> >> What cost function was used to arrive at the conclusion that its
> >> benefits outweigh its costs?
>
> Sorry this has taken a little while to reply to; I've been re-running my perf
> tests with the modern patches to recomfirm old data.

Thanks for the data!

> In terms of empirical evidence, I've run the kernel compilation benchmark (yes I
> know its a narrow use case, but I figure some data is better than no data), for
> all values of ANON_FOLIO_MAX_ORDER_UNHINTED {4k, 16k, 32k, 64k, 128k, 256k}.
>
> I've run each test 15 times across 5 system reboots on Ampere Altra (arm64),

What about x86 and ppc? Do we expect they might perform similarly wrt
different page sizes?

> with the kernel configured for 4K base pages - I could rerun for other base page
> sizes if we want to go further down this route.
>
> I've captured run time and peak memory usage, and taken the mean. The stdev for
> the peak memory usage is big-ish, but I'm confident this still captures the
> central tendancy well:
>
> | MAX_ORDER_UNHINTED | real-time | kern-time | user-time | peak memory |
> |:-------------------|------------:|------------:|------------:|:------------|
> | 4k | 0.0% | 0.0% | 0.0% | 0.0% |
> | 16k | -3.6% | -26.5% | -0.5% | -0.1% |
> | 32k | -4.8% | -37.4% | -0.6% | -0.1% |
> | 64k | -5.7% | -42.0% | -0.6% | -1.1% |
> | 128k | -5.6% | -42.1% | -0.7% | 1.4% |
> | 256k | -4.9% | -41.9% | -0.4% | 1.9% |
>
> 64K looks like the clear sweet spot to me.

Were the tests done under memory pressure? I agree 64KB might be a
reasonable value, but I don't think we can or need to make a
conclusion at this point: there are still pending questions from my
list.

Just to double check: we only need ANON_FOLIO_MAX_ORDER_UNHINTED
because of hugepage_vma_check(), is it correct?

> I know you have argued for using a page order in the past, rather than a size in
> bytes. But my argument is that user space is mostly doing mmaps based on sizes
> independent of the base page size (an assumption!) and a system's memory is
> obviously a fixed quantity that doesn't it doesn't change with base page size.
> So it feels more natural to limit internal fragmentation based on an absolute
> size rather than a quantity of pages. Kyril have also suggested using absolute
> sizes in the past [1].
>
> It's also worth mentioning that the file-backed memory "fault_around" mechanism
> chooses 64K.

This example actually is against your argument:
1. There have been multiple reports that fault around hurt
performances and had to be disabled for some workloads over the years
-- ANON_FOLIO_MAX_ORDER_UNHINTED is likely to cause regressions too.
2. Not only can fault around be disabled, its default value can be
changed too -- this series can't do either.
3. Most importantly, fault around does not do high-order allocations
-- this series does, and high-order allocations can be very difficult
under memory pressure.

> If this approach really looks unacceptable, I have a couple of other ideas. But
> I personally favour the approach that is already in the patch.

I understand. If the answer to my question above is yes, then let's
take a step back and figure out whether overloading existing ABIs is
acceptable or not. Does this sound good to you?

> 1) Add a large/small flag to arch_wants_pte_order(). arm64, at least, actually
> has 2 mechanisms, HPA and contpte. Currently arm64 is always returning the
> contpte order, but with a flag, it could return contpte order for large, and HPA
> order for small. (I know we previously passed the vma and we didn't like that,
> and this is pretty similar). I still think the SW (core-mm) needs a way to
> sensibly limit internal fragmentation though, so personally I still think having
> an upper limit in this case is useful.
>
> 2) More radical: move to a per-vma auto-tuning solution, which looks at the
> fault pattern and maintains an allocation order in the VMA, which is modified
> based on fault pattern. e.g. When we get faults that occur immediately adjacent
> to the allocated range, we increase; when we get faults not connected to
> previously allocated pages we decrease. I think it's an interesting thing to
> look at, but certainly prefer that it's not part of an MVP implementation.
>
> [1]
> https://lore.kernel.org/linux-mm/20230414140948.7pcaz6niyr2tpa7s@xxxxxxxxxxxxxxxxx/
>
>
> >>
> >>> + if ((vma->vm_flags & VM_NOHUGEPAGE) ||
> >>> + test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags) ||
> >>> + !hugepage_flags_enabled())
> >>> + order = 0;
> >>> + else {
> >>> + order = max(arch_wants_pte_order(), PAGE_ALLOC_COSTLY_ORDER);
> >>> +
> >>> + if (!hugepage_vma_check(vma, vma->vm_flags, false, true, true))
> >>> + order = min(order, ANON_FOLIO_MAX_ORDER_UNHINTED);
> >>> + }
> >
> > I'm a bit surprised to see the above: why can we overload existing
> > ABIs? I don't think we can.
>
> I think this is all covered by the conversation with David against v2; see [2]
> and proceeding replies. Argument is that VM_NOHUGEPAGE (and friends) is really a
> request from user space to optimize for the least memory wastage possible and
> avoid populating ptes that have not been expressly requested.
>
> [2]
> https://lore.kernel.org/linux-mm/524bacd2-4a47-2b8b-6685-c46e31a01631@xxxxxxxxxx/

Thanks for the info.

I think there might be a misunderstanding here.

David, can you please clarify whether you suggested we overland
(change the semantics) of existing ABIs?

This sounds like a big red flag to me. If that's really what you
suggest, can you shed some light on why this is acceptable to existing
userspace at all?

Thanks.