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

[Yin Fengwei]

On 8/1/23 14:36, Yu Zhao wrote:
> 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?
I will run the same text on Intel x86 platform.

Regards
Yin, Fengwei

> 
>> 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.