Re: [PATCH v8 03/10] mm: thp: Introduce multi-size THP sysfs interface

[David Hildenbrand]

On 05.12.23 10:50, Ryan Roberts wrote:
> On 05/12/2023 04:21, Barry Song wrote:
> > On Mon, Dec 4, 2023 at 11:21 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
> > > In preparation for adding support for anonymous multi-size THP,
> > > introduce new sysfs structure that will be used to control the new
> > > behaviours. A new directory is added under transparent_hugepage for each
> > > supported THP size, and contains an `enabled` file, which can be set to
> > > "inherit" (to inherit the global setting), "always", "madvise" or
> > > "never". For now, the kernel still only supports PMD-sized anonymous
> > > THP, so only 1 directory is populated.
> > >
> > > The first half of the change converts transhuge_vma_suitable() and
> > > hugepage_vma_check() so that they take a bitfield of orders for which
> > > the user wants to determine support, and the functions filter out all
> > > the orders that can't be supported, given the current sysfs
> > > configuration and the VMA dimensions. If there is only 1 order set in
> > > the input then the output can continue to be treated like a boolean;
> > > this is the case for most call sites. The resulting functions are
> > > renamed to thp_vma_suitable_orders() and thp_vma_allowable_orders()
> > > respectively.
> > >
> > > The second half of the change implements the new sysfs interface. It has
> > > been done so that each supported THP size has a `struct thpsize`, which
> > > describes the relevant metadata and is itself a kobject. This is pretty
> > > minimal for now, but should make it easy to add new per-thpsize files to
> > > the interface if needed in future (e.g. per-size defrag). Rather than
> > > keep the `enabled` state directly in the struct thpsize, I've elected to
> > > directly encode it into huge_anon_orders_[always|madvise|inherit]
> > > bitfields since this reduces the amount of work required in
> > > thp_vma_allowable_orders() which is called for every page fault.
> > >
> > > See Documentation/admin-guide/mm/transhuge.rst, as modified by this
> > > commit, for details of how the new sysfs interface works.
> > >
> > > Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> >
> > Reviewed-by: Barry Song <v-songbaohua@xxxxxxxx>
>
> Thanks!
>
> > > -khugepaged will be automatically started when
> > > -transparent_hugepage/enabled is set to "always" or "madvise, and it'll
> > > -be automatically shutdown if it's set to "never".
> > > +khugepaged will be automatically started when one or more hugepage
> > > +sizes are enabled (either by directly setting "always" or "madvise",
> > > +or by setting "inherit" while the top-level enabled is set to "always"
> > > +or "madvise"), and it'll be automatically shutdown when the last
> > > +hugepage size is disabled (either by directly setting "never", or by
> > > +setting "inherit" while the top-level enabled is set to "never").
> > >
> > >   Khugepaged controls
> > >   -------------------
> > >
> > > +.. note::
> > > +   khugepaged currently only searches for opportunities to collapse to
> > > +   PMD-sized THP and no attempt is made to collapse to other THP
> > > +   sizes.
> >
> > For small-size THP, collapse is probably a bad idea. we like a one-shot
> > try in Android especially we are using a 64KB and less large folio size. if
> > PF succeeds in getting large folios, we map large folios, otherwise we
> > give up as those memories can be quite unstably swapped-out, swapped-in
> > and madvised to be DONTNEED.
> >
> > too many compactions will increase power consumption and decrease UI
> > response.
>
> Understood; that's very useful information for the Android context. Multiple
> people have made comments about eventually needing khugepaged (or something
> similar) support in the server context though to async collapse to contpte size.
> Actually one suggestion was a user space daemon that scans and collapses with
> MADV_COLLAPSE. I suspect the key will be to ensure whatever solution we go for
> is flexible and can be enabled/disabled/configured for the different environments.

There certainly is interest for 2 MiB THP on arm64 64k where the THP 
size would normally be 512 MiB. In that scenario, khugepaged makes 
perfect sense.

--
Cheers,

David / dhildenb