linux-kernel - Re: [PATCH v6 2/4] KVM: X86: Add Paravirt TLB Shootdown

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Date:   Wed, 29 Nov 2017 17:21:19 +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>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v6 2/4] KVM: X86: Add Paravirt TLB Shootdown

2017-11-27 20:05-0800, 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.
> 
> 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@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> @@ -498,6 +498,37 @@ static void __init kvm_apf_trap_init(void)
>  	update_intr_gate(X86_TRAP_PF, async_page_fault);
>  }
>  
> +static DEFINE_PER_CPU(cpumask_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;
> +	cpumask_t *flushmask = &per_cpu(__pv_tlb_mask, smp_processor_id());
> +
> +	if (unlikely(!flushmask))
> +		return;

I don't see how this can be NULL and if it could, we'd have to call
native_flush_tlb_others() instead of returning anyway.

Also, Peter mentioned that we're wasting memory (default is 1k per CPU)
when not running on KVM.  Hyper-V hijacks x86_platform.apic_post_init()
to achieve late allocation.  smp_ops.smp_prepare_cpus seems slightly
better for our purposes, but I don't really like either.

Couldn't we use use arch_initcall(), or early_initcall() if there are
complications with allocating after smp_init()?

Thanks.