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

[Yu Zhao]

On Thu, Aug 3, 2023 at 8:16 PM Zi Yan <ziy@xxxxxxxxxx> wrote:
>
> On 3 Aug 2023, at 20:19, Yu Zhao wrote:
>
> > On Thu, Aug 3, 2023 at 8:27 AM Kirill A. Shutemov
> > <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> >>
> >> On Thu, Aug 03, 2023 at 01:43:31PM +0100, Ryan Roberts 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.
> >>>
> >>> So LAF must at least honour MADV_NOHUGEPAGE to prevent breaking existing use
> >>> cases. And once we do this, then I think the least confusing thing is for it to
> >>> also honor the "never" system/process state; so if either the system, process or
> >>> vma has explicitly opted-out of THP, then LAF should also be bypassed.
> >>>
> >>> Similarly, any case that would previously cause the allocation of PMD-sized THP
> >>> must continue to be honoured, else we risk performance regression.
> >>>
> >>> That leaves the "madvise/no-hint" case, and all THP fallback paths due to the
> >>> VMA not being correctly aligned or sized to hold a PMD-sized mapping. In these
> >>> cases, we will attempt to use LAF first, and fallback to single page if the vma
> >>> size/alignment doesn't permit it.
> >>>
> >>>                 | never     | madvise   | always
> >>> ----------------|-----------|-----------|-----------
> >>> no hint         | S         | LAF>S     | THP>LAF>S
> >>> MADV_HUGEPAGE   | S         | THP>LAF>S | THP>LAF>S
> >>> MADV_NOHUGEPAGE | S         | S         | S
> >>>
> >>> I think this (perhaps conservative) approach will be the least surprising to
> >>> users. And is the policy that is already implemented in this patch.
> >>
> >> This looks very reasonable.
> >>
> >> The only questionable field is no-hint/madvise. I can argue for both LAF>S
> >> and S here. I think LAF>S is fine as long as we are not too aggressive
> >> with allocation order.
> >>
> >> I think we need to work on eliminating reasons for users to set 'never'.
> >> If something behaves better with 'never' kernel has failed user.
> >>
> >>> Downsides of this policy
> >>> ------------------------
> >>>
> >>> As Yu and Yin have pointed out, there are some workloads which do not perform
> >>> well with THP, due to large fault latency or memory wastage, etc. But which
> >>> _may_ still benefit from LAF. By taking the conservative approach, we exclude
> >>> these workloads from benefiting automatically.
> >>
> >> Hm. I don't buy it. Why THP with order-9 is too much, but order-8 LAF is
> >> fine?
> >
> > No, it's not. And no one said order-8 LAF is fine :) The starting
> > order for LAF that we have been discussing is at most 64KB (vs 2MB
> > THP). For my taste, it's still too large. I'd go with 32KB/16KB.
>
> I guess it is because ARM64 supports contig PTE at 64KB, so getting
> large anon folio at 64KB on ARM64 would have an extra perf boost when
> set contig PTE bits patch is also in.
>
> On x86_64, 32KB might be better on AMD CPUs that support PTE clustering,
> which would use a single TLB entry for 8 contiguous 4KB pages and is
> done at microarchitecture level without additional software changes.
>
> >
> > However, the same argument can be used to argue against the policy
> > Ryan listed above: why order-10 LAF is ok for madvise but not order-11
> > (which becomes "always")?
> >
> > I'm strongly against this policy for two practical reasons I learned
> > from tuning THPs in our data centers:
>
> Do you mind writing down your policy? That would help us see and discuss
> the difference.
>
> > 1. By doing the above, we are blurring the lines between those values
> > and making real-world performance tuning extremely hard if not
> > impractice.
> > 2. As I previously pointed out: if we mix LAFs with THPs, we actually
> > risk causing performance regressions because giving smaller VMAs LAFs
> > can deprive large VMAs of THPs.
>
> I think these two reasons are based on that we do not have a reasonable
> LAF+THP allocation and management policy and we do not fully understand
> the pros and cons of using LAF and mixing LAF with THP. It would be
> safe to separate LAF and THP. By doing so,
>
> 1. for workloads do not benefit from THP, we can turn on LAF alone to
> see if there is a performance boost and further understand if LAF
> hurts, has no impactor , or improves the performance of these workloads.
>
> 2. for workloads benefit from THP, we can also turn on LAF separately
> to understand the performance impact of LAF (hurt, no change, or improve).

This is basically what I've been suggesting. We should have a separate
knob, not overload the existing ones. And this separate knob should be
able to take a list of fallback orders. After we have a wider
deployment, we might gain a better understanding of the "cost
function". Then we can try to build some in-kernel heuristics that
automatically decides the best orders to fallback. If/when we get
there, we can simply extend the knob by adding a new "magic word",
e.g., "auto".

> Ultimately, after we understand the performance impact of LAF, THP, and
> mix of them and come up a reasonable kernel policy, a unified knob would
> make sense. But we are not there yet.

Exactly.