linux-kernel - Re: [PATCH RESEND 2/3] KVM: Add paravirt remote TLB flush

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Date:   Thu, 9 Nov 2017 16:11:02 +0100
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH RESEND 2/3] KVM: Add paravirt remote TLB flush

2017-11-08 18:02-0800, Wanpeng Li:
> From: Wanpeng Li <wanpeng.li@...mail.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.
> 
> 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 <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
> 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;