Message-ID: <CAA1CXcD8FCdCsBkyW=Ppbr-ZRD8PNmPu-3khipX0fVK3mxs-EQ@mail.gmail.com>
Date: Wed, 21 May 2025 04:41:54 -0600
From: Nico Pache <npache@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
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Subject: Re: [PATCH v6 0/4] mm: introduce THP deferred setting

On Tue, May 20, 2025 at 3:43 AM Lorenzo Stoakes
<lorenzo.stoakes@...cle.com> wrote:
>
> On Wed, May 14, 2025 at 09:38:53PM -0600, Nico Pache wrote:
> > This series is a follow-up to [1], which adds mTHP support to khugepaged.
> > mTHP khugepaged support is a "loose" dependency for the sysfs/sysctl
> > configs to make sense. Without it global="defer" and  mTHP="inherit" case
> > is "undefined" behavior.
>
> How can this be a follow up to an unmerged series? I'm confused by that.
Hi Lorenzo,

follow up or loose dependency. Not sure the correct terminology.

Either way, as I was developing this as a potential solution for the
THP internal fragmentation issue, upstream was working on adding
mTHPs. By adding a new THP sysctl entry I noticed mTHP would now be
missing the same entry. Furthermore I was told mTHP support for
khugepaged was a desire, so I began working on it in conjunction. So
given the undefined behavior of defer globally while any mix of mTHP
settings, it became dependent on the khugepaged support. Either way
patch 1 of this series is the core functionality. The rest is to fill
the undefined behavior gap.
>
> And you're saying that you're introducing 'undefined behaviour' on the
> assumption that another series which seems to have quite a bit of
> discussion let to run will be merged?
This could technically get merged without the mTHP khugepaged changes,
but then the reviews would probably all be pointing out what I pointed
out above. Chicken or Egg problem...
>
> While I'd understand if this was an RFC just to put the idea out there,
> you're not proposing it as such?
Nope we've already discussed this in both the MM alignment and thp
upstream meetings, no one was opposing it, and a lot of testing was
done-- by me, RH's CI, and our perf teams. Ive posted several RFCs
before posting a patchset.
>
> Unless there's a really good reason we're doing this way (I may be missing
> something), can we just have this as an RFC until the series it depends on
> is settled?
Hopefully paragraph one clears this up! They were built in
conjunction, but posting them as one series didn't feel right (and
IIRC this was also discussed, and this was decided).
>
> >
> > We've seen cases were customers switching from RHEL7 to RHEL8 see a
> > significant increase in the memory footprint for the same workloads.
> >
> > Through our investigations we found that a large contributing factor to
> > the increase in RSS was an increase in THP usage.
> >
> > For workloads like MySQL, or when using allocators like jemalloc, it is
> > often recommended to set /transparent_hugepages/enabled=never. This is
> > in part due to performance degradations and increased memory waste.
> >
> > This series introduces enabled=defer, this setting acts as a middle
> > ground between always and madvise. If the mapping is MADV_HUGEPAGE, the
> > page fault handler will act normally, making a hugepage if possible. If
> > the allocation is not MADV_HUGEPAGE, then the page fault handler will
> > default to the base size allocation. The caveat is that khugepaged can
> > still operate on pages that are not MADV_HUGEPAGE.
> >
> > This allows for three things... one, applications specifically designed to
> > use hugepages will get them, and two, applications that don't use
> > hugepages can still benefit from them without aggressively inserting
> > THPs at every possible chance. This curbs the memory waste, and defers
> > the use of hugepages to khugepaged. Khugepaged can then scan the memory
> > for eligible collapsing. Lastly there is the added benefit for those who
> > want THPs but experience higher latency PFs. Now you can get base page
> > performance at the PF handler and Hugepage performance for those mappings
> > after they collapse.
> >
> > Admins may want to lower max_ptes_none, if not, khugepaged may
> > aggressively collapse single allocations into hugepages.
> >
> > TESTING:
> > - Built for x86_64, aarch64, ppc64le, and s390x
> > - selftests mm
> > - In [1] I provided a script [2] that has multiple access patterns
> > - lots of general use.
>
> OK so this truly is dependent on the unmerged series? Or isn't it?
>
> Is your testing based on that?
Most of the testing was done in conjunction, but independent testing
was also done on this series (including by a large customer that was
itching to try the changes, and they were very satisfied with the
results).
>
> Because again... that surely makes this series a no-go until we land the
> prior (which might be changed, and thus necessitate re-testing).
>
> Are you going to provide any of these numbers/data anywhere?
There is a link to the results in this cover letter
[3] - https://people.redhat.com/npache/mthp_khugepaged_defer/testoutput2/output.html
>
> > - redis testing. This test was my original case for the defer mode. What I
> >    was able to prove was that THP=always leads to increased max_latency
> >    cases; hence why it is recommended to disable THPs for redis servers.
> >    However with 'defer' we dont have the max_latency spikes and can still
> >    get the system to utilize THPs. I further tested this with the mTHP
> >    defer setting and found that redis (and probably other jmalloc users)
> >    can utilize THPs via defer (+mTHP defer) without a large latency
> >    penalty and some potential gains. I uploaded some mmtest results
> >    here[3] which compares:
> >        stock+thp=never
> >        stock+(m)thp=always
> >        khugepaged-mthp + defer (max_ptes_none=64)
> >
> >   The results show that (m)THPs can cause some throughput regression in
> >   some cases, but also has gains in other cases. The mTHP+defer results
> >   have more gains and less losses over the (m)THP=always case.
> >
> > V6 Changes:
> > - nits
> > - rebased dependent series and added review tags
> >
> > V5 Changes:
> > - rebased dependent series
> > - added reviewed-by tag on 2/4
> >
> > V4 Changes:
> > - Minor Documentation fixes
> > - rebased the dependent series [1] onto mm-unstable
> >     commit 0e68b850b1d3 ("vmalloc: use atomic_long_add_return_relaxed()")
> >
> > V3 Changes:
> > - Combined the documentation commits into one, and moved a section to the
> >   khugepaged mthp patchset
> >
> > V2 Changes:
> > - base changes on mTHP khugepaged support
> > - Fix selftests parsing issue
> > - add mTHP defer option
> > - add mTHP defer Documentation
> >
> > [1] - https://lore.kernel.org/all/20250515032226.128900-1-npache@redhat.com/
> > [2] - https://gitlab.com/npache/khugepaged_mthp_test
> > [3] - https://people.redhat.com/npache/mthp_khugepaged_defer/testoutput2/output.html
> >
> > Nico Pache (4):
> >   mm: defer THP insertion to khugepaged
> >   mm: document (m)THP defer usage
> >   khugepaged: add defer option to mTHP options
> >   selftests: mm: add defer to thp setting parser
> >
> >  Documentation/admin-guide/mm/transhuge.rst | 31 +++++++---
> >  include/linux/huge_mm.h                    | 18 +++++-
> >  mm/huge_memory.c                           | 69 +++++++++++++++++++---
> >  mm/khugepaged.c                            |  8 +--
> >  tools/testing/selftests/mm/thp_settings.c  |  1 +
> >  tools/testing/selftests/mm/thp_settings.h  |  1 +
> >  6 files changed, 106 insertions(+), 22 deletions(-)
> >
> > --
> > 2.49.0
> >
>

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