On Thu, Feb 13, 2025 at 7:02 PM Dev Jain <dev.jain@xxxxxxx> wrote: > > > > On 14/02/25 1:09 am, Nico Pache wrote: > > On Thu, Feb 13, 2025 at 1:26 AM Dev Jain <dev.jain@xxxxxxx> wrote: > >> > >> > >> > >> On 12/02/25 10:19 pm, Nico Pache wrote: > >>> On Tue, Feb 11, 2025 at 5:50 AM Dev Jain <dev.jain@xxxxxxx> wrote: > >>>> > >>>> > >>>> > >>>> On 11/02/25 6:00 am, Nico Pache wrote: > >>>>> The following series provides khugepaged and madvise collapse with the > >>>>> capability to collapse regions to mTHPs. > >>>>> > >>>>> To achieve this we generalize the khugepaged functions to no longer depend > >>>>> on PMD_ORDER. Then during the PMD scan, we keep track of chunks of pages > >>>>> (defined by MTHP_MIN_ORDER) that are utilized. This info is tracked > >>>>> using a bitmap. After the PMD scan is done, we do binary recursion on the > >>>>> bitmap to find the optimal mTHP sizes for the PMD range. The restriction > >>>>> on max_ptes_none is removed during the scan, to make sure we account for > >>>>> the whole PMD range. max_ptes_none will be scaled by the attempted collapse > >>>>> order to determine how full a THP must be to be eligible. If a mTHP collapse > >>>>> is attempted, but contains swapped out, or shared pages, we dont perform the > >>>>> collapse. > >>>>> > >>>>> With the default max_ptes_none=511, the code should keep its most of its > >>>>> original behavior. To exercise mTHP collapse we need to set max_ptes_none<=255. > >>>>> With max_ptes_none > HPAGE_PMD_NR/2 you will experience collapse "creep" and > >>>>> constantly promote mTHPs to the next available size. > >>>>> > >>>>> Patch 1: Some refactoring to combine madvise_collapse and khugepaged > >>>>> Patch 2: Refactor/rename hpage_collapse > >>>>> Patch 3-5: Generalize khugepaged functions for arbitrary orders > >>>>> Patch 6-9: The mTHP patches > >>>>> > >>>>> --------- > >>>>> Testing > >>>>> --------- > >>>>> - Built for x86_64, aarch64, ppc64le, and s390x > >>>>> - selftests mm > >>>>> - I created a test script that I used to push khugepaged to its limits while > >>>>> monitoring a number of stats and tracepoints. The code is available > >>>>> here[1] (Run in legacy mode for these changes and set mthp sizes to inherit) > >>>>> The summary from my testings was that there was no significant regression > >>>>> noticed through this test. In some cases my changes had better collapse > >>>>> latencies, and was able to scan more pages in the same amount of time/work, > >>>>> but for the most part the results were consistant. > >>>>> - redis testing. I tested these changes along with my defer changes > >>>>> (see followup post for more details). > >>>>> - some basic testing on 64k page size. > >>>>> - lots of general use. These changes have been running in my VM for some time. > >>>>> > >>>>> Changes since V1 [2]: > >>>>> - Minor bug fixes discovered during review and testing > >>>>> - removed dynamic allocations for bitmaps, and made them stack based > >>>>> - Adjusted bitmap offset from u8 to u16 to support 64k pagesize. > >>>>> - Updated trace events to include collapsing order info. > >>>>> - Scaled max_ptes_none by order rather than scaling to a 0-100 scale. > >>>>> - No longer require a chunk to be fully utilized before setting the bit. Use > >>>>> the same max_ptes_none scaling principle to achieve this. > >>>>> - Skip mTHP collapse that requires swapin or shared handling. This helps prevent > >>>>> some of the "creep" that was discovered in v1. > >>>>> > >>>>> [1] - https://gitlab.com/npache/khugepaged_mthp_test > >>>>> [2] - https://lore.kernel.org/lkml/20250108233128.14484-1-npache@xxxxxxxxxx/ > >>>>> > >>>>> Nico Pache (9): > >>>>> introduce khugepaged_collapse_single_pmd to unify khugepaged and > >>>>> madvise_collapse > >>>>> khugepaged: rename hpage_collapse_* to khugepaged_* > >>>>> khugepaged: generalize hugepage_vma_revalidate for mTHP support > >>>>> khugepaged: generalize alloc_charge_folio for mTHP support > >>>>> khugepaged: generalize __collapse_huge_page_* for mTHP support > >>>>> khugepaged: introduce khugepaged_scan_bitmap for mTHP support > >>>>> khugepaged: add mTHP support > >>>>> khugepaged: improve tracepoints for mTHP orders > >>>>> khugepaged: skip collapsing mTHP to smaller orders > >>>>> > >>>>> include/linux/khugepaged.h | 4 + > >>>>> include/trace/events/huge_memory.h | 34 ++- > >>>>> mm/khugepaged.c | 422 +++++++++++++++++++---------- > >>>>> 3 files changed, 306 insertions(+), 154 deletions(-) > >>>>> > >>>> > >>>> Does this patchset suffer from the problem described here: > >>>> https://lore.kernel.org/all/8abd99d5-329f-4f8d-8680-c2d48d4963b6@xxxxxxx/ > >>> Hi Dev, > >>> > >>> Sorry I meant to get back to you about that. > >>> > >>> I understand your concern, but like I've mentioned before, the scan > >>> with the read lock was done so we dont have to do the more expensive > >>> locking, and could still gain insight into the state. You are right > >>> that this info could become stale if the state changes dramatically, > >>> but the collapse_isolate function will verify it and not collapse. > >> > >> If the state changes dramatically, the _isolate function will verify it, > >> and fallback. And this fallback happens after following this costly > >> path: retrieve a large folio from the buddy allocator -> swapin pages > >> from the disk -> mmap_write_lock() -> anon_vma_lock_write() -> TLB flush > >> on all CPUs -> fallback in _isolate(). > >> If you do fail in _isolate(), doesn't it make sense to get the updated > >> state for the next fallback order immediately, because we have prior > >> information that we failed because of PTE state? What your algorithm > >> will do is *still* follow the costly path described above, and again > >> fail in _isolate(), instead of failing in hpage_collapse_scan_pmd() like > >> mine would. > > > > You do raise a valid point here, I can optimize my solution by > > detecting certain collapse failure types and jump to the next scan. > > I'll add that to my solution, thanks! > > > > As for the disagreement around the bitmap, we'll leave that up to the > > community to decide since we have differing opinions/solutions. > > > >> > >> The verification of the PTE state by the _isolate() function is the "no > >> turning back" point of the algorithm. The verification by > >> hpage_collapse_scan_pmd() is the "let us see if proceeding is even worth > >> it, before we do costly operations" point of the algorithm. > >> > >>> From my testing I found this to rarely happen. > >> > >> Unfortunately, I am not very familiar with performance testing/load > >> testing, I am fairly new to kernel programming, so I am getting there. > >> But it really depends on the type of test you are running, what actually > >> runs on memory-intensive systems, etc etc. In fact, on loaded systems I > >> would expect the PTE state to dramatically change. But still, no opinion > >> here. > > > > Yeah there are probably some cases where it happens more often. > > Probably in cases of short lived allocations, but khugepaged doesn't > > run that frequently so those won't be that big of an issue. > > > > Our performance team is currently testing my implementation so I > > should have more real workload test results soon. The redis testing > > had some gains and didn't show any signs of obvious regressions. > > > > As for the testing, check out > > https://gitlab.com/npache/khugepaged_mthp_test/-/blob/master/record-khuge-performance.sh?ref_type=heads > > this does the tracing for my testing script. It can help you get > > started. There are 3 different traces being applied there: the > > bpftrace for collapse latencies, the perf record for the flamegraph > > (not actually that useful, but may be useful to visualize any > > weird/long paths that you may not have noticed), and the trace-cmd > > which records the tracepoint of the scan and the collapse functions > > then processes the data using the awk script-- the output being the > > scan rate, the pages collapsed, and their result status (grouped by > > order). > > > > You can also look into https://github.com/gormanm/mmtests for > > testing/comparing kernels. I was running the > > config-memdb-redis-benchmark-medium workload. > > Thanks. I'll take a look. > > > > >> > >>> > >>> Also, khugepaged, my changes, and your changes are all a victim of > >>> this. Once we drop the read lock (to either allocate the folio, or > >>> right before acquiring the write_lock), the state can change. In your > >>> case, yes, you are gathering more up to date information, but is it > >>> really that important/worth it to retake locks and rescan for each > >>> instance if we are about to reverify with the write lock taken? > >> > >> You said "reverify": You are removing the verification, so this step > >> won't be reverification, it will be verification. We do not want to > >> verify *after* we have already done 95% of latency-heavy stuff, only to > >> know that we are going to fail. > >> > >> Algorithms in the kernel, in general, are of the following form: 1) > >> Verify if a condition is true, resulting in taking a control path -> 2) > >> do a lot of stuff -> "no turning back" step, wherein before committing > >> (by taking locks, say), reverify if this is the control path we should > >> be in. You are eliminating step 1). > >> > >> Therefore, I will have to say that I disagree with your approach. > >> > >> On top of this, in the subjective analysis in [1], point number 7 (along > >> with point number 1) remains. And, point number 4 remains. > > > > for 1) your worst case of 1024 is not the worst case. There are 8 > > possible orders in your implementation, if all are enabled, that is > > 4096 iterations in the worst case. > > Yes, that is exactly what I wrote in 1). I am still not convinced that > the overhead you produce + 512 iterations is going to beat 4096 > iterations. Anyways, that is hand-waving and we should test this. > > > This becomes WAY worse on 64k page size, ~45,000 iterations vs 4096 in my case. > > Sorry, I am missing something here; how does the number of iterations > change with page size? Am I not scanning the PTE table, which is > invariant to the page size? I got the calculation wrong the first time and it's actually worst. Lets hope I got this right this time on ARM64 64k kernel: PMD size = 512M PTE= 64k PTEs per PMD = 8192 log2(8192) = 13 - 2 = 11 number of (m)THP sizes including PMD (the first and second order are skipped) Assuming I understand your algorithm correctly, in the worst case you are scanning the whole PMD for each order. So you scan 8192 PTEs 11 times. 8192 * 11 = 90112. Please let me know if I'm missing something here. > > >> > >> [1] > >> https://lore.kernel.org/all/23023f48-95c6-4a24-ac8b-aba4b1a441b4@xxxxxxx/ > >> > >>> > >>> So in my eyes, this is not a "problem" > >> > >> Looks like the kernel scheduled us for a high-priority debate, I hope > >> there's no deadlock :) > >> > >>> > >>> Cheers, > >>> -- Nico > >>> > >>> > >>>> > >>> > >> > > >
