linux-kernel - Re: [Patch v4 03/18] KVM: x86/mmu: Track count of pages in KVM MMU page caches globally

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Date:   Thu, 9 Mar 2023 05:18:11 +0000
From:   Mingwei Zhang <mizhang@...gle.com>
To:     Vipin Sharma <vipinsh@...gle.com>
Cc:     Zhi Wang <zhi.wang.linux@...il.com>, seanjc@...gle.com,
        pbonzini@...hat.com, bgardon@...gle.com, dmatlack@...gle.com,
        jmattson@...gle.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [Patch v4 03/18] KVM: x86/mmu: Track count of pages in KVM MMU
 page caches globally

> >
> > 1) Previously mmu_topup_memory_caches() works fine without a lock.
> > 2) IMHO I was suspecting if this lock seems affects the parallelization
> > of the TDP MMU fault handling.
> >
> > TDP MMU fault handling is intend to be optimized for parallelization fault
> > handling by taking a read lock and operating the page table via atomic
> > operations. Multiple fault handling can enter the TDP MMU fault path
> > because of read_lock(&vcpu->kvm->mmu_lock) below.
> >
> > W/ this lock, it seems the part of benefit of parallelization is gone
> > because the lock can contend earlier above. Will this cause performance
> > regression?
> 
> This is a per vCPU lock, with this lock each vCPU will still be able
> to perform parallel fault handling without contending for lock.
> 

I am curious how effective it is by trying to accquiring this per vCPU
lock? If a vcpu thread should stay within the (host) kernel (vmx
root/non-root) for the vast majority of the time, isn't the shrinker
always fail to make any progress?