On 17/01/2023 10:29, Usama Arif wrote:
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 is well integrated in core kernel code and can be used to improve performance in locking (owner_on_cpu usage in mutex_spin_on_owner, mutex_can_spin_on_owner, rtmutex_spin_on_owner and osq_lock) and scheduling (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 significant improvement on overcommitted hosts (vCPUs > pCPUS), as waiting for preempted vCPUs reduces performance.
Hi, Just wanted to check if there are any comments for this? Thanks, Usama
If merged, vcpu_is_preempted could also be used to optimize IPI performance (along with directed yield to target IPI vCPU) similar to how its done in x86 (https://lore.kernel.org/all/1560255830-8656-2-git-send-email-wanpengli@xxxxxxxxxxx/) 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+vcpu_is_preempted) and without (6.0 base) the patchset. TestName 6.0 base 6.0+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 278% 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 kernel. This can be seen from one of the callstack of the perf output of the shell scripts benchmark on 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 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+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 base. Hence the patches have no negative affect when vCPUs < pCPUs. The respective QEMU change to test this is at https://github.com/uarif1/qemu/commit/2da2c2927ae8de8f03f439804a0dad9cf68501b6. Looking forward to your response! Thanks, Usama --- v2->v3 - Updated the patchset from 6.0 to 6.2-rc3 - Made pv_lock_init an early_initcall - Improved documentation - Changed pvlock_vcpu_state to aligned struct - Minor improvevments 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 | 54 +++++++++ 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 | 15 +++ arch/arm64/include/asm/spinlock.h | 16 ++- arch/arm64/include/uapi/asm/kvm.h | 3 + arch/arm64/kernel/paravirt.c | 113 ++++++++++++++++++ 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/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 + 20 files changed, 403 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
