This patchset adds support for vcpu_is_preempted in arm64, which allows the guest to check if a vcpu was scheduled out, which is useful to know incase it was holding a lock. vcpu_is_preempted can be used to improve performance in locking (see owner_on_cpu usage in mutex_spin_on_owner, mutex_can_spin_on_owner, rtmutex_spin_on_owner and osq_lock) and scheduling (see available_idle_cpu which is used in several places in kernel/sched/fair.c for e.g. in wake_affine to determine which CPU can run soonest): This patchset shows improvement on overcommitted hosts (vCPUs > pCPUS), as waiting for preempted vCPUs reduces performance. This patchset is inspired from the para_steal_clock implementation and from the work originally done by Zengruan Ye: https://lore.kernel.org/linux-arm-kernel/20191226135833.1052-1-yezengruan@xxxxxxxxxx/. All the results in the below experiments are done on an aws r6g.metal instance which has 64 pCPUs. The following table shows the index results of UnixBench running on a 128 vCPU VM with (6.0.0+vcpu_is_preempted) and without (6.0.0 base) the patchset. TestName 6.0.0 base 6.0.0+vcpu_is_preempted % improvement for vcpu_is_preempted Dhrystone 2 using register variables 187761 191274.7 1.871368389 Double-Precision Whetstone 96743.6 98414.4 1.727039308 Execl Throughput 689.3 10426 1412.548963 File Copy 1024 bufsize 2000 maxblocks 549.5 3165 475.978162 File Copy 256 bufsize 500 maxblocks 400.7 2084.7 420.2645371 File Copy 4096 bufsize 8000 maxblocks 894.3 5003.2 459.4543218 Pipe Throughput 76819.5 78601.5 2.319723508 Pipe-based Context Switching 3444.8 13414.5 289.4130283 Process Creation 301.1 293.4 -2.557289937 Shell Scripts (1 concurrent) 1248.1 28300.6 2167.494592 Shell Scripts (8 concurrent) 781.2 26222.3 3256.669227 System Call Overhead 3426 3729.4 8.855808523 System Benchmarks Index Score 3053 11534 277.7923354 This shows a 277% overall improvement using these patches. The biggest improvement is in the shell scripts benchmark, which forks a lot of processes. This acquires rwsem lock where a large chunk of time is spent in base 6.0.0 kernel. This can be seen from one of the callstack of the perf output of the shell scripts benchmark on 6.0.0 base (pseudo NMI enabled for perf numbers below): - 33.79% el0_svc - 33.43% do_el0_svc - 33.43% el0_svc_common.constprop.3 - 33.30% invoke_syscall - 17.27% __arm64_sys_clone - 17.27% __do_sys_clone - 17.26% kernel_clone - 16.73% copy_process - 11.91% dup_mm - 11.82% dup_mmap - 9.15% down_write - 8.87% rwsem_down_write_slowpath - 8.48% osq_lock Just under 50% of the total time in the shell script benchmarks ends up being spent in osq_lock in the base 6.0.0 kernel: Children Self Command Shared Object Symbol 17.19% 10.71% sh [kernel.kallsyms] [k] osq_lock 6.17% 4.04% sort [kernel.kallsyms] [k] osq_lock 4.20% 2.60% multi. [kernel.kallsyms] [k] osq_lock 3.77% 2.47% grep [kernel.kallsyms] [k] osq_lock 3.50% 2.24% expr [kernel.kallsyms] [k] osq_lock 3.41% 2.23% od [kernel.kallsyms] [k] osq_lock 3.36% 2.15% rm [kernel.kallsyms] [k] osq_lock 3.28% 2.12% tee [kernel.kallsyms] [k] osq_lock 3.16% 2.02% wc [kernel.kallsyms] [k] osq_lock 0.21% 0.13% looper [kernel.kallsyms] [k] osq_lock 0.01% 0.00% Run [kernel.kallsyms] [k] osq_lock and this comes down to less than 1% total with 6.0.0+vcpu_is_preempted kernel: Children Self Command Shared Object Symbol 0.26% 0.21% sh [kernel.kallsyms] [k] osq_lock 0.10% 0.08% multi. [kernel.kallsyms] [k] osq_lock 0.04% 0.04% sort [kernel.kallsyms] [k] osq_lock 0.02% 0.01% grep [kernel.kallsyms] [k] osq_lock 0.02% 0.02% od [kernel.kallsyms] [k] osq_lock 0.01% 0.01% tee [kernel.kallsyms] [k] osq_lock 0.01% 0.00% expr [kernel.kallsyms] [k] osq_lock 0.01% 0.01% looper [kernel.kallsyms] [k] osq_lock 0.00% 0.00% wc [kernel.kallsyms] [k] osq_lock 0.00% 0.00% rm [kernel.kallsyms] [k] osq_lock To make sure, there is no change in performance when vCPUs < pCPUs, UnixBench was run on a 32 CPU VM. The kernel with vcpu_is_preempted implemented performed 0.9% better overall than base kernel, and the individual benchmarks were within +/-2% improvement over 6.0.0 base. Hence the patches have no negative affect when vCPUs < pCPUs. The other method discussed in https://lore.kernel.org/linux-arm-kernel/20191226135833.1052-1-yezengruan@xxxxxxxxxx/ was pv conditional yield by Marc Zyngier and Will Deacon to reduce vCPU reschedule if the vCPU will exit immediately. (https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/log/?h=kvm-arm64/pvcy). The patches were ported to 6.0.0 kernel and tested with UnixBench with a 128 vCPU VM: TestName 6.0.0 base 6.0.0+pvcy % improvement for pvcy Dhrystone 2 using register variables 187761 183128 -2.467498575 Double-Precision Whetstone 96743.6 96645 -0.101918887 Execl Throughput 689.3 1019.8 47.9471928 File Copy 1024 bufsize 2000 maxblocks 549.5 2029.7 269.3721565 File Copy 256 bufsize 500 maxblocks 400.7 1439.4 259.2213626 File Copy 4096 bufsize 8000 maxblocks 894.3 3434.1 283.9986582 Pipe Throughput 76819.5 74268.8 -3.320380893 Pipe-based Context Switching 3444.8 5963.3 73.11019508 Process Creation 301.1 163.2 -45.79873796 Shell Scripts (1 concurrent) 1248.1 1859.7 49.00248378 Shell Scripts (8 concurrent) 781.2 1171 49.89759345 System Call Overhead 3426 3194.4 -6.760070053 System Benchmarks Index Score 3053 4605 50.83524402 pvcy shows a smaller overall improvement (50%) compared to vcpu_is_preempted (277%). Host side flamegraph analysis shows that ~60% of the host time when using pvcy is spent in kvm_handle_wfx, compared with ~1.5% when using vcpu_is_preempted, hence vcpu_is_preempted shows a larger improvement. It might be that pvcy can be used in combination with vcpu_is_preempted, but this series is to start a discussion on vcpu_is_preempted as it shows a much bigger improvement in performance and its much easier to review vcpu_is_preempted standalone. The respective QEMU change to test this is at https://github.com/uarif1/qemu/commit/2da2c2927ae8de8f03f439804a0dad9cf68501b6, Looking forward to your response! Thanks, Usama --- RFC->v2 - Fixed table and code referencing in pvlock documentation - Switched to using a single hypercall similar to ptp_kvm and made check for has_kvm_pvlock simpler Usama Arif (6): KVM: arm64: Document PV-lock interface KVM: arm64: Add SMCCC paravirtualised lock calls KVM: arm64: Support pvlock preempted via shared structure KVM: arm64: Provide VCPU attributes for PV lock KVM: arm64: Support the VCPU preemption check KVM: selftests: add tests for PV time specific hypercall Documentation/virt/kvm/arm/hypercalls.rst | 3 + Documentation/virt/kvm/arm/index.rst | 1 + Documentation/virt/kvm/arm/pvlock.rst | 52 ++++++++ Documentation/virt/kvm/devices/vcpu.rst | 25 ++++ arch/arm64/include/asm/kvm_host.h | 25 ++++ arch/arm64/include/asm/paravirt.h | 2 + arch/arm64/include/asm/pvlock-abi.h | 17 +++ arch/arm64/include/asm/spinlock.h | 16 ++- arch/arm64/include/uapi/asm/kvm.h | 3 + arch/arm64/kernel/paravirt.c | 112 ++++++++++++++++++ arch/arm64/kernel/setup.c | 3 + arch/arm64/kvm/Makefile | 2 +- arch/arm64/kvm/arm.c | 8 ++ arch/arm64/kvm/guest.c | 9 ++ arch/arm64/kvm/hypercalls.c | 8 ++ arch/arm64/kvm/pvlock.c | 100 ++++++++++++++++ include/linux/arm-smccc.h | 8 ++ include/linux/cpuhotplug.h | 1 + include/uapi/linux/kvm.h | 2 + tools/arch/arm64/include/uapi/asm/kvm.h | 1 + tools/include/linux/arm-smccc.h | 8 ++ .../selftests/kvm/aarch64/hypercalls.c | 2 + 22 files changed, 406 insertions(+), 2 deletions(-) create mode 100644 Documentation/virt/kvm/arm/pvlock.rst create mode 100644 arch/arm64/include/asm/pvlock-abi.h create mode 100644 arch/arm64/kvm/pvlock.c -- 2.25.1
