linux-kernel - Re: [PATCH] KVM: VMX: Optimize flushing the PML buffer

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Message-ID: <20210204225144.GU6468@xz-x1>
Date:   Thu, 4 Feb 2021 17:51:44 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Ben Gardon <bgardon@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Makarand Sonare <makarandsonare@...gle.com>,
        Kai Huang <kai.huang@...el.com>
Subject: Re: [PATCH] KVM: VMX: Optimize flushing the PML buffer

Hi, Ben,

On Thu, Feb 04, 2021 at 02:19:59PM -0800, Ben Gardon wrote:
> The average time for each run demonstrated a strange bimodal distribution,
> with clusters around 2 seconds and 2.5 seconds. This may have been a
> result of vCPU migration between NUMA nodes.

Have you thought about using numactl or similar technique to verify your idea
(force both vcpu threads binding, and memory allocations)?

>From the numbers it already shows improvements indeed, but just curious since
you raised this up. :)

> @@ -5707,13 +5708,18 @@ static void vmx_flush_pml_buffer(struct kvm_vcpu *vcpu)
>  	else
>  		pml_idx++;
>  
> +	memslots = kvm_vcpu_memslots(vcpu);
> +
>  	pml_buf = page_address(vmx->pml_pg);
>  	for (; pml_idx < PML_ENTITY_NUM; pml_idx++) {
> +		struct kvm_memory_slot *memslot;
>  		u64 gpa;
>  
>  		gpa = pml_buf[pml_idx];
>  		WARN_ON(gpa & (PAGE_SIZE - 1));
> -		kvm_vcpu_mark_page_dirty(vcpu, gpa >> PAGE_SHIFT);
> +
> +		memslot = __gfn_to_memslot(memslots, gpa >> PAGE_SHIFT);
> +		mark_page_dirty_in_slot(vcpu->kvm, memslot, gpa >> PAGE_SHIFT);

Since at it: make "gpa >> PAGE_SHIFT" a temp var too?

Thanks,

-- 
Peter Xu