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: <CADrL8HV5M-n72KDseDKWpGrUVMjC147Jqz98PxyG2ZeRVbFu8g@mail.gmail.com>
Date: Thu, 25 Jul 2024 14:55:30 -0700
From: James Houghton <jthoughton@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>, Paolo Bonzini <pbonzini@...hat.com>
Cc: 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>, Sean Christopherson <seanjc@...gle.com>, 
	Shaoqin Huang <shahuang@...hat.com>, Suzuki K Poulose <suzuki.poulose@....com>, 
	Wei Xu <weixugc@...gle.com>, Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>, 
	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 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).

It would be really nice to motivate this patch with a test that didn't
require patching the kernel... Oliver and Marc, please let me know if
you'd like to see more data. I'm also happy to simply drop this patch.