Hi, I'm sending this RFC mainly to get some early feedback on the approach used for implementing "Eager Page Splitting" on ARM. "Eager Page Splitting" improves the performance of dirty-logging (used in live migrations) when guest memory is backed by huge-pages. It's an optimization used in Google Cloud since 2016 on x86, and for the last couple of months on ARM. I tried multiple ways of implementing this optimization on ARM: from completely reusing the stage2 mapper, to implementing a new walker from scratch, and some versions in between. This RFC is one of those in between. They all have similar performance benefits, based on some light performance testing (mainly dirty_log_perf_test). Background and motivation ========================= Dirty logging is typically used for live-migration iterative copying. KVM implements dirty-logging at the PAGE_SIZE granularity (will refer to 4K pages from now on). It does it by faulting on write-protected 4K pages. Therefore, enabling dirty-logging on a huge-page requires breaking it into 4K pages in the first place. KVM does this breaking on fault, and because it's in the critical path it only maps the 4K page that faulted; every other 4K page is left unmapped. This is not great for performance on ARM for a couple of reasons: - Splitting on fault can halt vcpus for milliseconds in some implementations. Splitting a block PTE requires using a broadcasted TLB invalidation (TLBI) for every huge-page (due to the break-before-make requirement). Note that x86 doesn't need this. We observed some implementations that take millliseconds to complete broadcasted TLBIs when done in parallel from multiple vcpus. And that's exactly what happens when doing it on fault: multiple vcpus fault at the same time triggering TLBIs in parallel. - Read intensive guest workloads end up paying for dirty-logging. Only mapping the faulting 4K page means that all the other pages that were part of the huge-page will now be unmapped. The effect is that any access, including reads, now has to fault. Eager Page Splitting (on ARM) ============================= Eager Page Splitting fixes the above two issues by eagerly splitting huge-pages when enabling dirty logging. The goal is to avoid doing it while faulting on write-protected pages. This is what the TDP MMU does for x86 [0], except that x86 does it for different reasons: to avoid grabbing the MMU lock on fault. Note that taking care of write-protection faults still requires grabbing the MMU lock on ARM, but not on x86 (with the fast_page_fault path). An additional benefit of eagerly splitting huge-pages is that it can be done in a controlled way (e.g., via an IOCTL). This series provides two knobs for doing it, just like its x86 counterpart: when enabling dirty logging, and when using the KVM_CLEAR_DIRTY_LOG ioctl. The benefit of doing it on KVM_CLEAR_DIRTY_LOG is that this ioctl takes ranges, and not complete memslots like when enabling dirty logging. This means that the cost of splitting (mainly broadcasted TLBIs) can be throttled: split a range, wait for a bit, split another range, etc. The benefits of this approach were presented by Oliver Upton at KVM Forum 2022 [1]. Implementation ============== Patches 1-4 add a pgtable utility function for splitting huge block PTEs: kvm_pgtable_stage2_split(). Patches 5-6 add support for not doing break-before-make on huge-page breaking when FEAT_BBM level 2 is supported. Patches 7-11 add support for eagerly splitting huge-pages when enabling dirty-logging and when using the KVM_CLEAR_DIRTY_LOG ioctl. Note that this is just like what x86 does, and the code is actually based on it. And finally, patch 12: KVM: arm64: Use local TLBI on permission relaxation adds support for using local TLBIs instead of broadcasts when doing permission relaxation. This last patch is key to achieving good performance during dirty-logging, as eagerly breaking huge-pages replaces mapping new pages with permission relaxation. Got this patch (indirectly) from Marc Z. and took the liberty of adding a commit message. Note: this applies on top of kvmarm/next at 3ba3e7266ab6. Although most of the tests were done using [2] (v6.1-rc1 + [3]). Performance evaluation ====================== The performance benefits were tested on an Ampere AmpereOne using the dirty_log_perf_test selftest with 2M huge-pages. The first test uses a write-only sequential workload where the stride is 2M instead of 4K [2]. The idea with this experiment is to emulate a random access pattern writing a different huge-page at every access. Observe that the benefit increases with the number of vcpus: up to 5.76x for 152 vcpus. ./dirty_log_perf_test_sparse -s anonymous_hugetlb_2mb -b 1G -v $i -i 3 -m 2 +-------+----------+---------------+ | vCPUs | next | next + series | | | (ms) | (ms) | +-------+----------+---------------+ | 1 | 3.34 | 1.85 | | 2 | 3.51 | 1.92 | | 4 | 3.93 | 1.99 | | 8 | 5.78 | 1.97 | | 16 | 10.06 | 2.08 | | 32 | 21.07 | 3.06 | | 64 | 41.75 | 6.92 | | 128 | 86.09 | 12.07 | | 152 | 109.72 | 18.94 | +-------+----------+---------------+ This second test measures the benefit of eager page splitting on read intensive workloads (1 write for every 10 reads). As in the other test, the benefit increases with the number of vcpus, up to 8.82x for 152 vcpus. ./dirty_log_perf_test -s anonymous_hugetlb_2mb -b 1G -v $i -i 3 -m 2 -f 10 +-------+----------+---------------+ | vCPUs | next | next + series | | | (sec) | (sec) | +-------+----------+---------------+ | 1 | 0.65 | 0.09 | | 2 | 0.70 | 0.09 | | 4 | 0.71 | 0.09 | | 8 | 0.72 | 0.10 | | 16 | 0.76 | 0.10 | | 32 | 1.61 | 0.15 | | 64 | 3.46 | 0.36 | | 128 | 5.49 | 0.74 | | 152 | 6.44 | 0.73 | +-------+----------+---------------+ Thanks, Ricardo [0] https://lore.kernel.org/kvm/20220119230739.2234394-1-dmatlack@xxxxxxxxxx/ [1] https://kvmforum2022.sched.com/event/15jJq/kvmarm-at-scale-improvements-to-the-mmu-in-the-face-of-hardware-growing-pains-oliver-upton-google [2] https://github.com/ricarkol/linux/commit/f78e9102b2bff4fb7f30bee810d7d611a537b46d [3] https://lore.kernel.org/kvmarm/20221107215644.1895162-1-oliver.upton@xxxxxxxxx/ Marc Zyngier (1): KVM: arm64: Use local TLBI on permission relaxation Ricardo Koller (11): KVM: arm64: Relax WARN check in stage2_make_pte() KVM: arm64: Allow visiting block PTEs in post-order KVM: arm64: Add stage2_create_removed() KVM: arm64: Add kvm_pgtable_stage2_split() arm64: Add a capability for FEAT_BBM level 2 KVM: arm64: Split block PTEs without using break-before-make KVM: arm64: Refactor kvm_arch_commit_memory_region() KVM: arm64: Add kvm_uninit_stage2_mmu() KVM: arm64: Split huge pages when dirty logging is enabled KVM: arm64: Open-code kvm_mmu_write_protect_pt_masked() KVM: arm64: Split huge pages during KVM_CLEAR_DIRTY_LOG arch/arm64/include/asm/esr.h | 1 + arch/arm64/include/asm/kvm_arm.h | 1 + arch/arm64/include/asm/kvm_asm.h | 4 + arch/arm64/include/asm/kvm_host.h | 30 ++++ arch/arm64/include/asm/kvm_mmu.h | 1 + arch/arm64/include/asm/kvm_pgtable.h | 33 ++++- arch/arm64/kernel/cpufeature.c | 11 ++ arch/arm64/kvm/hyp/nvhe/hyp-main.c | 10 ++ arch/arm64/kvm/hyp/nvhe/setup.c | 2 +- arch/arm64/kvm/hyp/nvhe/tlb.c | 54 +++++++ arch/arm64/kvm/hyp/pgtable.c | 205 +++++++++++++++++++++++++-- arch/arm64/kvm/hyp/vhe/tlb.c | 32 +++++ arch/arm64/kvm/mmu.c | 177 +++++++++++++++++++---- arch/arm64/tools/cpucaps | 1 + 14 files changed, 517 insertions(+), 45 deletions(-) -- 2.38.1.431.g37b22c650d-goog _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
