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

[Wanpeng Li]

2017-11-30 14:01 GMT+08:00 Wanpeng Li <kernellwp@xxxxxxxxx>:
> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> ---
>  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
>