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

[Yu Zhao]

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.

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:
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.