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

From: Yin, Fengwei
Date: Wed Aug 02 2023 - 09:51:24 EST



On 8/2/2023 4:02 PM, Ryan Roberts wrote:
> On 01/08/2023 07:36, 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?
>
> It's my assumption that they should behave similarly, but I haven't actually
> tested it. Thanks to Yin Fengwei for the kind offer to run these tests on x86!
>
> Yin: I have a test tool that will automate running this and gather up the
> results. Not sure if this is useful to you? I can share if you want? I also
> slightly modified the code to add a boot param to set the
> ANON_FOLIO_MAX_ORDER_UNHINTED value, and the test tool automatically injected
> the values.
Not necessary. I started to run the test. I suppose I could share the test result
tomorrow.


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?
>
> No. I have the capability to run these tests in a memcg with limited max memory
> though to force swap. I planned to do some sweeps increasing memory pressure,
> all for ANON_FOLIO_MAX_ORDER_UNHINTED=64k. Doing this for all values above will
> take too much time, but I could do them at a single value of max memory if that
> helps? I'm not sure how I would choose that single value though? (probably do
> the sweep for 64k then choose a sensible point on that graph?).
>
> 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.
>
> You mean there are still pending questions from your list above, or you have
> others that you havent asked yet? If the former, I've answered all of the above
> to the best of my ability. My view is that this value is always going to be
> tuned empirically so its difficult to answer with absolutes. What can I do to
> improve confidence? If the latter, then please let me know your other questions.
>
>>
>> Just to double check: we only need ANON_FOLIO_MAX_ORDER_UNHINTED
>> because of hugepage_vma_check(), is it correct?
>
> tldr; yes, correct.
>
> My problem is that for arm64 16k and 64k base page configs, the contpte size is
> 2M. It's my view that this is too big to allocate when it has not been
> explicitly asked for. And I think experience with THP (which is 2M for 4K
> systems today) demonstrates that. But I would still like to benefit from reduced
> SW overhead where possible (i.e. reduced page faults), and I would still like to
> use the contpte 2M mappings when the user has signalled that they can tolerate
> the potential internal fragmentation (MADV_HUGEPAGE).
>
> In practical terms, ANON_FOLIO_MAX_ORDER_UNHINTED does not affect arm64/4K at
> all (because the contpte size is 64K) and it does not impact the other 4K base
> page arches, which don't currently implement arch_wants_pte_order() meaning they
> get the default PAGE_ALLOC_COSTLY_ORDER=3=32k.
>
>>
>>> 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.
>
> I don't see how ANON_FOLIO_MAX_ORDER_UNHINTED can cause regressions; it's adding
> a limit making the behaviour of Large Anon Folios more similar to the old
> behaviour than it otherwise would be. Without it, we will be allocating 2M
> folios in some cases which would be much more likely to cause regression in
> unprepared apps IMHO.
>
>> 2. Not only can fault around be disabled, its default value can be
>> changed too -- this series can't do either.
>
> I had a mechanism for that in the previous version, but discussion concluded
> that we should avoid adding the control for now and add it only if/when we have
> identified a workload that will benefit.
>
>> 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.
>
> But ANON_FOLIO_MAX_ORDER_UNHINTED *reduces* the order from what it otherwise
> would be. So I don't see how its making things worse?
>
>>
>>> 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?
>
> Yes, good idea. Hopefully my explanation above (and all the previous threads)
> gives you a good idea for the problem as I see it, and how I think hooking the
> THP hints is helpful to the solution. If I've understood David's previuous
> remarks correctly, then this also aligns with David's opinions (David you could
> confirm/deny this please?). Could you explain your position that hooking these
> ABIs is a bad approach?
>
>>
>>> 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.
>