Message-ID: <CAA1CXcAfbzwr7QC6JngPvTPtBSf=6WfnhVn+gK2rHMrSTuS8vw@mail.gmail.com>
Date: Wed, 28 May 2025 22:26:25 -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 Wed, May 21, 2025 at 5:25 AM Lorenzo Stoakes
<lorenzo.stoakes@...cle.com> wrote:
>
> To start with I do apologise for coming to this at v6, I realise it's
> irritating to have push back at this late stage. This is more so my attempt
> to understand where this series -sits- so I can properly review it.

No worries at all! The only thing that frustrates/upsets me in
upstream mailing lists is unprovoked rudeness (which you have not
been).
>
> So please bear with me here :)
>
> So, I remain very confused. This may just be a _me_ thing here :)
>
> So let me check my understanding:
>
> 1. This series introduces this new THP deferred mode.
> 2. By 'follow-up' really you mean 'inspired by' or 'related to' right?
> 3. If this series lands before [1], commits 2 - 4 are 'undefined
>    behaviour'.
The khugepaged mTHP support should land first as without it, adding a
defer option to the global parameters, makes for undefined behavior in
the sysctls from a admin perspective.
>
> In my view if 3 is true this series should be RFC until [1] merges.
Ideally I was trying to get them merged together (Andrew actually had
them both in mm-new a few weeks ago, but a bug was found that got it
pulled, but that is fixed now). The series' complement each other
nicely.
>
> If I've got it wrong and this needs to land first, we should RFC [1].
The khugepaged series [1] should get merged first, but I was shooting
for both at the same time.
>
> That way we can un-RFC once the dependency is met.
>
> We have about 5 [m]THP series in flight at the moment, all touching at
> least vaguely related stuff, so any help for reviewers would be hugely
> appreciated thanks :)
>
> On Wed, May 21, 2025 at 04:41:54AM -0600, Nico Pache wrote:
> > 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.
> >
>
> See above. Let's nail this down please.
>
> > 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).
>
> 'This was also discussed and this was decided' :)
>
> I'm guessing rather you mean discussion was had with other reviewers and of
> course our earstwhile THP maintainer David, and you guys decided this made
> more sense?
>
> Obviously upstream discussion is what counts, but as annoying as it is, one
> does have to address the concerns of reviewers even if late to a series
> (again, apologies for this).
>
> So, to be clear - I'm not intending to hold up or block the series, I just
> want to understand how things are, this is the purpose here.

Thanks I do appreciate the discussion around the process as I am
fairly new to upstream work (at least to this magnitude). I have been
mostly downstream focused for the last 6 years and I'm trying to shift
upstream as much as possible. So please bear with me as I learn all
the minor undocumented caveats!
>
> Thanks!
>
> > >
> > > >
> > > > 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).
>
> You should make this very clear in the cover letter.
I will try to do better at updating and providing more information in
my cover letters and patches. I was never sure how much information to
include! I guess the more the merrier.
>
> > >
> > > 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
> > >
>
> Ultimately it's not ok in mm to have a link to a website that might go away
> any time, these cover letters are 'baked in' to the commit log. Are you
> sure this website with 'testoutput2' will exist in 10 years time? :)
>
> You should at the very least add a summary of this data in the cover
> letter, perhaps referring back to this link as 'at the time of writing full
> results are available at' or something like this.

Ok good to know I will find a way to summarize the performance and
result changes more cleanly in the cover letter.
>
> > > > - 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
> > > >
> > >
> >
>
> Cheers, Lorenzo
>

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