Re: [PATCH v7 2/4] KVM: X86: Add Paravirt TLB Shootdown

Wanpeng Li
2017-11-30T07:53:07Z

2017-11-30 14:01 GMT+08:00 Wanpeng Li <kernellwp@gmail.com>:
> From: Wanpeng Li <wanpeng.li@hotmail.com>
>
> Remote flushing api's does a busy wait which is fine in bare-metal
> scenario. But with-in the guest, the vcpus might have been pre-empted
> or blocked. In this scenario, the initator vcpu would end up
> busy-waiting for a long amount of time.
>
> This patch set implements para-virt flush tlbs making sure that it
> does not wait for vcpus that are sleeping. And all the sleeping vcpus
> flush the tlb on guest enter.
>
> The best result is achieved when we're overcommiting the host by running
> multiple vCPUs on each pCPU. In this case PV tlb flush avoids touching
> vCPUs which are not scheduled and avoid the wait on the main CPU.
>
> Testing on a Xeon Gold 6142 2.6GHz 2 sockets, 32 cores, 64 threads,
> so 64 pCPUs, and each VM is 64 vCPUs.
>
> ebizzy -M
> vanilla optimized boost
> 1VM 46799 48670 4%
> 2VM 23962 42691 78%
> 3VM 16152 37539 132%
>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
> ---
> Documentation/virtual/kvm/cpuid.txt | 4 +++
> arch/x86/include/uapi/asm/kvm_para.h | 2 ++
> arch/x86/kernel/kvm.c | 47 ++++++++++++++++++++++++++++++++++++
> 3 files changed, 53 insertions(+)
>
> diff --git a/Documentation/virtual/kvm/cpuid.txt b/Documentation/virtual/kvm/cpuid.txt
> index 3c65feb..dcab6dc 100644
> --- a/Documentation/virtual/kvm/cpuid.txt
> +++ b/Documentation/virtual/kvm/cpuid.txt
> @@ -54,6 +54,10 @@ KVM_FEATURE_PV_UNHALT || 7 || guest checks this feature bit
> || || before enabling paravirtualized
> || || spinlock support.
> ------------------------------------------------------------------------------
> +KVM_FEATURE_PV_TLB_FLUSH || 9 || guest checks this feature bit
> + || || before enabling paravirtualized
> + || || tlb flush.
> +------------------------------------------------------------------------------
> KVM_FEATURE_CLOCKSOURCE_STABLE_BIT || 24 || host will warn if no guest-side
> || || per-cpu warps are expected in
> || || kvmclock.
> diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
> index 763b692..8fbcc16 100644
> --- a/arch/x86/include/uapi/asm/kvm_para.h
> +++ b/arch/x86/include/uapi/asm/kvm_para.h
> @@ -25,6 +25,7 @@
> #define KVM_FEATURE_STEAL_TIME 5
> #define KVM_FEATURE_PV_EOI 6
> #define KVM_FEATURE_PV_UNHALT 7
> +#define KVM_FEATURE_PV_TLB_FLUSH 9
>
> /* The last 8 bits are used to indicate how to interpret the flags field
> * in pvclock structure. If no bits are set, all flags are ignored.
> @@ -53,6 +54,7 @@ struct kvm_steal_time {
>
> #define KVM_VCPU_NOT_PREEMPTED (0 << 0)
> #define KVM_VCPU_PREEMPTED (1 << 0)
> +#define KVM_VCPU_SHOULD_FLUSH (1 << 1)
>
> #define KVM_CLOCK_PAIRING_WALLCLOCK 0
> struct kvm_clock_pairing {
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 6610b92..64fb9a4 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -498,6 +498,34 @@ static void __init kvm_apf_trap_init(void)
> update_intr_gate(X86_TRAP_PF, async_page_fault);
> }
>
> +static DEFINE_PER_CPU(cpumask_var_t, __pv_tlb_mask);
> +
> +static void kvm_flush_tlb_others(const struct cpumask *cpumask,
> + const struct flush_tlb_info *info)
> +{
> + u8 state;
> + int cpu;
> + struct kvm_steal_time *src;
> + struct cpumask *flushmask = this_cpu_cpumask_var_ptr(__pv_tlb_mask);
> +
> + cpumask_copy(flushmask, cpumask);
> + /*
> + * We have to call flush only on online vCPUs. And
> + * queue flush_on_enter for pre-empted vCPUs
> + */
> + for_each_cpu(cpu, flushmask) {
> + src = &per_cpu(steal_time, cpu);
> + state = READ_ONCE(src->preempted);
> + if ((state & KVM_VCPU_PREEMPTED)) {
> + if (try_cmpxchg(&src->preempted, &state,
> + state | KVM_VCPU_SHOULD_FLUSH))
> + __cpumask_clear_cpu(cpu, flushmask);
> + }
> + }
> +
> + native_flush_tlb_others(flushmask, info);
> +}
> +
> static void __init kvm_guest_init(void)
> {
> int i;
> @@ -517,6 +545,9 @@ static void __init kvm_guest_init(void)
> pv_time_ops.steal_clock = kvm_steal_clock;
> }
>
> + if (kvm_para_has_feature(KVM_FEATURE_PV_TLB_FLUSH))
> + pv_mmu_ops.flush_tlb_others = kvm_flush_tlb_others;
> +
> if (kvm_para_has_feature(KVM_FEATURE_PV_EOI))
> apic_set_eoi_write(kvm_guest_apic_eoi_write);
>
> @@ -598,6 +629,22 @@ static __init int activate_jump_labels(void)
> }
> arch_initcall(activate_jump_labels);
>
> +static __init int kvm_setup_pv_tlb_flush(void)
> +{
> + int cpu;
> +
> + if (kvm_para_has_feature(KVM_FEATURE_PV_TLB_FLUSH)) {
> + for_each_possible_cpu(cpu) {
> + zalloc_cpumask_var_node(per_cpu_ptr(&__pv_tlb_mask, cpu),
> + GFP_KERNEL, cpu_to_node(cpu));
> + }
> + pr_info("KVM setup remote TLB flush\n");

Please change it to "KVM setup pv remote TLB flush\n" if it is the
last version before applying. :)

Regards,
Wanpeng Li

> + }
> +
> + return 0;
> +}
> +arch_initcall(kvm_setup_pv_tlb_flush);
> +
> #ifdef CONFIG_PARAVIRT_SPINLOCKS
>
> /* Kick a cpu by its apicid. Used to wake up a halted vcpu */
> --
> 2.7.4
>