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

[Yin, Fengwei]

On 8/5/2023 5:58 AM, Zi Yan wrote:
> On 4 Aug 2023, at 17:30, David Hildenbrand wrote:
> 
>> On 04.08.23 23:26, Yu Zhao wrote:
>>> On Fri, Aug 4, 2023 at 3:13 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>>>>
>>>> On 04.08.23 23:00, Yu Zhao wrote:
>>>>> On Fri, Aug 4, 2023 at 2:23 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>>>>>>
>>>>>> On 04.08.23 10:27, Ryan Roberts wrote:
>>>>>>> On 04/08/2023 00:50, Yu Zhao wrote:
>>>>>>>> On Thu, Aug 3, 2023 at 6:43 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>>>>>>>>>
>>>>>>>>> + Kirill
>>>>>>>>>
>>>>>>>>> On 26/07/2023 10:51, Ryan Roberts 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.
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> ...
>>>>>>>>>
>>>>>>>>>> +#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.
>>>>>>>>>> +      */
>>>>>>>>>> +
>>>>>>>>>> +     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);
>>>>>>>>>> +     }
>>>>>>>>>> +
>>>>>>>>>> +     return order;
>>>>>>>>>> +}
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Hi All,
>>>>>>>>>
>>>>>>>>> I'm writing up the conclusions that we arrived at during discussion in the THP
>>>>>>>>> meeting yesterday, regarding linkage with exiting THP ABIs. It would be great if
>>>>>>>>> I can get explicit "agree" or disagree + rationale from at least David, Yu and
>>>>>>>>> Kirill.
>>>>>>>>>
>>>>>>>>> In summary; I think we are converging on the approach that is already coded, but
>>>>>>>>> I'd like confirmation.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> The THP situation today
>>>>>>>>> -----------------------
>>>>>>>>>
>>>>>>>>>     - At system level: THP can be set to "never", "madvise" or "always"
>>>>>>>>>     - At process level: THP can be "never" or "defer to system setting"
>>>>>>>>>     - At VMA level: no-hint, MADV_HUGEPAGE, MADV_NOHUGEPAGE
>>>>>>>>>
>>>>>>>>> That gives us this table to describe how a page fault is handled, according to
>>>>>>>>> process state (columns) and vma flags (rows):
>>>>>>>>>
>>>>>>>>>                    | never     | madvise   | always
>>>>>>>>> ----------------|-----------|-----------|-----------
>>>>>>>>> no hint         | S         | S         | THP>S
>>>>>>>>> MADV_HUGEPAGE   | S         | THP>S     | THP>S
>>>>>>>>> MADV_NOHUGEPAGE | S         | S         | S
>>>>>>>>>
>>>>>>>>> Legend:
>>>>>>>>> S       allocate single page (PTE-mapped)
>>>>>>>>> LAF     allocate lage anon folio (PTE-mapped)
>>>>>>>>> THP     allocate THP-sized folio (PMD-mapped)
>>>>>>>>>>          fallback (usually because vma size/alignment insufficient for folio)
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Principles for Large Anon Folios (LAF)
>>>>>>>>> --------------------------------------
>>>>>>>>>
>>>>>>>>> David tells us there are use cases today (e.g. qemu live migration) which use
>>>>>>>>> MADV_NOHUGEPAGE to mean "don't fill any PTEs that are not explicitly faulted"
>>>>>>>>> and these use cases will break (i.e. functionally incorrect) if this request is
>>>>>>>>> not honoured.
>>>>>>>>
>>>>>>>> I don't remember David saying this. I think he was referring to UFFD,
>>>>>>>> not MADV_NOHUGEPAGE, when discussing what we need to absolutely
>>>>>>>> respect.
>>>>>>>
>>>>>>> My understanding was that MADV_NOHUGEPAGE was being applied to regions *before*
>>>>>>> UFFD was being registered, and the app relied on MADV_NOHUGEPAGE to not back any
>>>>>>> unfaulted pages. It's not completely clear to me how not honouring
>>>>>>> MADV_NOHUGEPAGE would break things though. David?
>>>>>>
>>>>>> Sorry, I'm still lagging behind on some threads.
>>>>>>
>>>>>> Imagine the following for VM postcopy live migration:
>>>>>>
>>>>>> (1) Set MADV_NOHUGEPAGE on guest memory and discard all memory (e.g.,
>>>>>>        MADV_DONTNEED), to start with a clean slate.
>>>>>> (2) Migrates some pages during precopy from the source and stores them
>>>>>>        into guest memory on the destination. Some of the memory locations
>>>>>>        will have pages populated.
>>>>>> (3) At some point, decide to enable postcopy: enable userfaultfd on
>>>>>>        guest memory.
>>>>>> (4) Discard *selected* pages again that have been dirtied in the
>>>>>>        meantime on the source. These are pages that have been migrated
>>>>>>        previously.
>>>>>> (5) Start running the VM on the destination.
>>>>>> (6) Anything that's not populated will trigger userfaultfd missing
>>>>>>        faults. Then, you can request them from the source and place them.
>>>>>>
>>>>>> Assume you would populate more than required during 2), you can end up
>>>>>> not getting userfaultfd faults during 4) and corrupt your guest state.
>>>>>> It works if during (2) you migrated all guest memory, or if during 4)
>>>>>> you zap everything that still needs migr
>>>>>
>>>>> I see what you mean now. Thanks.
>>>>>
>>>>> Yes, in this case we have to interpret MADV_NOHUGEPAGE as nothing >4KB.
>>>>
>>>> Note that it's still even unclear to me why we want to *not* call these
>>>> things THP. It would certainly make everything less confusing if we call
>>>> them THP, but with additional attributes.
>>>>
>>>> I think that is one of the first things we should figure out because it
>>>> also indirectly tells us what all these toggles mean and how/if we
>>>> should redefine them (and if they even apply).
>>>>
>>>> Currently THP == PMD size
>>>>
>>>> In 2016, Hugh already envisioned PUD/PGD THP (see 49920d28781d ("mm:
>>>> make transparent hugepage size public")) when he explicitly exposed
>>>> "hpage_pmd_size". Not "hpage_size".
>>>>
>>>> For hugetlb on arm64 we already support various sizes that are < PMD
>>>> size and *not* call them differently. It's a huge(tlb) page. Sometimes
>>>> we refer to them as cont-PTE hugetlb pages.
>>>>
>>>>
>>>> So, nowadays we do have "PMD-sized THP", someday we might have
>>>> "PUD-sized THP". Can't we come up with a name to describe sub-PMD THP?
>>>>
>>>> Is it really of value if we invent a new term for them? Yes, I was not
>>>> enjoying "Flexible THP".
>>>>
>>>>
>>>> Once we figured that out, we should figure out if MADV_HUGEPAGE meant
>>>> "only PMD-sized THP" or anything else?
>>>>
>>>> Also, we can then figure out if MADV_NOHUGEPAGE meant "only PMD-sized
>>>> THP" or anything else?
>>>>
>>>>
>>>> The simplest approach to me would be "they imply any THP, and once we
>>>> need more tunables we might add some", similar to what Kirill also raised.
>>>>
>>>>
>>>> Again, it's all unclear to me at this point and I'm happy to hear
>>>> opinions, because I really don't know.
>>>
>>> I agree these points require more discussion. But I don't think we
>>> need to conclude them now, unless they cause correctness issues like
>>> ignoring MADV_NOHUGEPAGE would. My concern is that if we decide to go
>>> with "they imply any THP" and *expose this to userspace now*, we might
>>> regret later.
>>
>> If we don't think they are THP, probably MADV_NOHUGEPAGE should not apply and we should be ready to find other ways to deal with the mess we eventually create. If we want to go down that path, sure.
>>
>> If they are THP, to me there is not really a question if MADV_NOHUGEPAGE applies to them or not. Unless we want to build a confusing piece of software ;)
> 
> I think it is good to call them THP, since they are transparent huge (>order-0) pages.
> But the concern is that before we have a reasonable management policy for order>0 &&
> order<9 THPs, mixing them with existing order-9 THP might give user unexpected
> performance outcome. Unless we are sure they will always performance improvement,
> we might repeat the old THP path, namely users begin to disable THP by default
> to avoid unexpected performance hiccup. That is the reason Yu wants to separate
> LAF from THP at the moment.
> 
> Maybe call it THP (experimental) for now and merge it to THP when we have a stable
> policy. For knobs, we might add "any-order" to the existing "never", "madvise"
> and another interface to specify max hinted order (enforcing <9) for "any-order".
> Later, we can allow users to specify any max hinted order, including 9. Just an
> idea.
I suspect that all the config knobs (enable/disable mixing mode, define "any-order"
or "specific-order") will be exist long term. Because there are always new workloads
need be tuned against these configs.


Regards
Yin, Fengwei

> 
> 
> --
> Best Regards,
> Yan, Zi