linux-kernel - Re: [PATCH v6 03/11] KVM: arm64: Relax locking for kvm_test_age_gfn and kvm_age

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Message-ID: <CAOUHufYc3hr-+fp14jgEkDN++v6t-z-PRf1yQdKtnje6SgLiiA@mail.gmail.com>
Date: Fri, 16 Aug 2024 19:03:27 -0600
From: Yu Zhao <yuzhao@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: James Houghton <jthoughton@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Paolo Bonzini <pbonzini@...hat.com>, Ankit Agrawal <ankita@...dia.com>, 
	Axel Rasmussen <axelrasmussen@...gle.com>, Catalin Marinas <catalin.marinas@....com>, 
	David Matlack <dmatlack@...gle.com>, David Rientjes <rientjes@...gle.com>, 
	James Morse <james.morse@....com>, Jason Gunthorpe <jgg@...pe.ca>, Jonathan Corbet <corbet@....net>, 
	Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>, 
	Raghavendra Rao Ananta <rananta@...gle.com>, Ryan Roberts <ryan.roberts@....com>, 
	Shaoqin Huang <shahuang@...hat.com>, Suzuki K Poulose <suzuki.poulose@....com>, 
	Wei Xu <weixugc@...gle.com>, Will Deacon <will@...nel.org>, Zenghui Yu <yuzenghui@...wei.com>, 
	kvmarm@...ts.linux.dev, kvm@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v6 03/11] KVM: arm64: Relax locking for kvm_test_age_gfn
 and kvm_age_gfn

On Fri, Aug 16, 2024 at 6:46 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Thu, Jul 25, 2024, James Houghton wrote:
> > On Tue, Jul 23, 2024 at 6:11 PM James Houghton <jthoughton@...gle.com> wrote:
> > >
> > > Replace the MMU write locks (taken in the memslot iteration loop) for
> > > read locks.
> > >
> > > Grabbing the read lock instead of the write lock is safe because the
> > > only requirement we have is that the stage-2 page tables do not get
> > > deallocated while we are walking them. The stage2_age_walker() callback
> > > is safe to race with itself; update the comment to reflect the
> > > synchronization change.
> > >
> > > Signed-off-by: James Houghton <jthoughton@...gle.com>
> > > ---
> >
> > Here is some data to show that this patch at least *can* be helpful:
> >
> > # arm64 patched to do aging (i.e., set HAVE_KVM_MMU_NOTIFIER_YOUNG_FAST_ONLY)
> > # The test is faulting memory in while doing aging as fast as possible.
> > # taskset -c 0-32 ./access_tracking_perf_test -l -r /dev/cgroup/memory
> > -p -v 32 -m 3
> >
> > # Write lock
> > vcpu wall time                : 3.039207157s
> > lru_gen avg pass duration     : 1.660541541s, (passes:2, total:3.321083083s)
> >
> > # Read lock
> > vcpu wall time                : 3.010848445s
> > lru_gen avg pass duration     : 0.306623698s, (passes:11, total:3.372860688s)
> >
> > Aging is able to run significantly faster, but vCPU runtime isn't
> > affected much (in this test).
>
> Were you expecting vCPU runtime to improve (more)?  If so, lack of movement could
> be due to KVM arm64 taking mmap_lock for read when handling faults:
>
> https://lore.kernel.org/all/Zr0ZbPQHVNzmvwa6@google.com

For the above test, I don't think it's mmap_lock -- the reclaim path,
e.g., when zswapping guest memory, has two stages: aging (scanning
PTEs) and eviction (unmapping PTEs). Only testing the former isn't
realistic at all. IOW, for a r/w lock use case, only testing the read
lock path would be bad coverage.