On Tue, May 20, 2025 at 3:43 AM Lorenzo Stoakes <lorenzo.stoa...@oracle.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-npa...@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 > > >
