2017-11-08 18:02-0800, Wanpeng Li:
> From: Wanpeng Li <[email protected]>
>
> 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.
>
> Test on a Haswell i7 desktop 4 cores (2HT), so 8 pCPUs, running ebizzy in
> one linux guest.
>
> ebizzy -M
> vanilla optimized boost
> 8 vCPUs 10152 10083 -0.68%
> 16 vCPUs 1224 4866 297.5%
> 24 vCPUs 1109 3871 249%
> 32 vCPUs 1025 3375 229.3%
>
> Cc: Paolo Bonzini <[email protected]>
> Cc: Radim Krčmář <[email protected]>
> Signed-off-by: Wanpeng Li <[email protected]>
> ---
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> @@ -465,6 +465,33 @@ static void __init kvm_apf_trap_init(void)
> update_intr_gate(X86_TRAP_PF, async_page_fault);
> }
>
> +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;
> + cpumask_t flushmask;
> +
> +
> + 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, cpumask) {
> + src = &per_cpu(steal_time, cpu);
> + state = src->preempted;
> + if ((state & KVM_VCPU_PREEMPTED)) {
> + if (cmpxchg(&src->preempted, state, state | 1 <<
> + KVM_VCPU_SHOULD_FLUSH))
We won't be flushing unless the last argument reads 'state |
KVM_VCPU_SHOULD_FLUSH' and the result will be the original value that
should be compared with state to avoid a race that would drop running
VCPU:
if (cmpxchg(&src->preempted, state, state | KVM_VCPU_SHOULD_FLUSH) == state)
> + cpumask_clear_cpu(cpu, &flushmask);
> + }
> + }
> +
> + native_flush_tlb_others(&flushmask, info);
> +}
> +
> void __init kvm_guest_init(void)
> {
> int i;
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