On Wed, May 21, 2025 at 5:25 AM Lorenzo Stoakes <lorenzo.stoa...@oracle.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.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. > > > > 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-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 > > > > > > > > > > > Cheers, Lorenzo >
