From: Sean Christopherson <seanjc@google•com>
To: Yu Zhao <yuzhao@google•com>
Cc: James Houghton <jthoughton@google•com>,
kvm@vger•kernel.org, linux-doc@vger•kernel.org,
Catalin Marinas <catalin.marinas@arm•com>,
Atish Patra <atishp@atishpatra•org>,
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linux-kselftest@vger•kernel.org,
Raghavendra Rao Ananta <rananta@google•com>,
linux-riscv@lists•infradead.org, Shuah Khan <shuah@kernel•org>,
Jonathan Corbet <corbet@lwn•net>,
Anup Patel <anup@brainfault•org>,
Huacai Chen <chenhuacai@kernel•org>,
David Rientjes <rientjes@google•com>,
Zenghui Yu <yuzenghui@huawei•com>,
Axel Rasmussen <axelrasmussen@google•com>,
linux-mips@vger•kernel.org, Albert Ou <aou@eecs•berkeley.edu>,
Ryan Roberts <ryan.roberts@arm•com>,
Will Deacon <will@kernel•org>,
Suzuki K Poulose <suzuki.poulose@arm•com>,
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Ankit Agrawal <ankita@nvidia•com>,
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Tianrui Zhao <zhaotianrui@loongson•cn>,
linuxppc-dev@lists•ozlabs.org
Subject: Re: [PATCH v4 2/7] mm: multi-gen LRU: Have secondary MMUs participate in aging
Date: Wed, 29 May 2024 14:59:55 -0700 [thread overview]
Message-ID: <ZlelW93_T6P-ZuSZ@google.com> (raw)
In-Reply-To: <CAOUHufYFHKLwt1PWp2uS6g174GZYRZURWJAmdUWs5eaKmhEeyQ@mail.gmail.com>
On Wed, May 29, 2024, Yu Zhao wrote:
> On Wed, May 29, 2024 at 12:05 PM James Houghton <jthoughton@google•com> wrote:
> >
> > Secondary MMUs are currently consulted for access/age information at
> > eviction time, but before then, we don't get accurate age information.
> > That is, pages that are mostly accessed through a secondary MMU (like
> > guest memory, used by KVM) will always just proceed down to the oldest
> > generation, and then at eviction time, if KVM reports the page to be
> > young, the page will be activated/promoted back to the youngest
> > generation.
>
> Correct, and as I explained offline, this is the only reasonable
> behavior if we can't locklessly walk secondary MMUs.
>
> Just for the record, the (crude) analogy I used was:
> Imagine a large room with many bills ($1, $5, $10, ...) on the floor,
> but you are only allowed to pick up 10 of them (and put them in your
> pocket). A smart move would be to survey the room *first and then*
> pick up the largest ones. But if you are carrying a 500 lbs backpack,
> you would just want to pick up whichever that's in front of you rather
> than walk the entire room.
>
> MGLRU should only scan (or lookaround) secondary MMUs if it can be
> done lockless. Otherwise, it should just fall back to the existing
> approach, which existed in previous versions but is removed in this
> version.
IIUC, by "existing approach" you mean completely ignore secondary MMUs that don't
implement a lockless walk?
next prev parent reply other threads:[~2024-05-29 22:06 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-05-29 18:05 [PATCH v4 0/7] mm: multi-gen LRU: Walk secondary MMU page tables while aging James Houghton
2024-05-29 18:05 ` [PATCH v4 1/7] mm/Kconfig: Add LRU_GEN_WALKS_SECONDARY_MMU James Houghton
2024-05-29 18:05 ` [PATCH v4 2/7] mm: multi-gen LRU: Have secondary MMUs participate in aging James Houghton
2024-05-29 21:03 ` Yu Zhao
2024-05-29 21:59 ` Sean Christopherson [this message]
2024-05-29 22:21 ` Yu Zhao
2024-05-29 22:58 ` Sean Christopherson
2024-05-30 1:08 ` James Houghton
2024-05-31 6:05 ` Yu Zhao
2024-05-31 7:02 ` Oliver Upton
2024-05-31 16:45 ` Yu Zhao
2024-05-31 18:41 ` Oliver Upton
2024-06-03 22:45 ` James Houghton
2024-06-03 23:03 ` Sean Christopherson
2024-06-03 23:16 ` James Houghton
2024-06-04 0:23 ` Sean Christopherson
2024-05-31 7:24 ` Oliver Upton
2024-05-31 20:31 ` Yu Zhao
2024-05-31 21:06 ` David Matlack
2024-05-31 21:09 ` David Matlack
2024-05-31 21:18 ` Oliver Upton
2024-05-29 18:05 ` [PATCH v4 3/7] KVM: Add lockless memslot walk to KVM James Houghton
2024-05-29 21:51 ` Sean Christopherson
2024-05-30 3:26 ` James Houghton
2024-05-29 18:05 ` [PATCH v4 4/7] KVM: Move MMU lock acquisition for test/clear_young to architecture James Houghton
2024-05-29 21:55 ` Sean Christopherson
2024-05-30 3:27 ` James Houghton
2024-05-29 18:05 ` [PATCH v4 5/7] KVM: x86: Relax locking for kvm_test_age_gfn and kvm_age_gfn James Houghton
2024-05-29 18:05 ` [PATCH v4 6/7] KVM: arm64: " James Houghton
2024-05-31 19:11 ` Oliver Upton
2024-05-31 19:18 ` Oliver Upton
2024-06-04 22:20 ` James Houghton
2024-06-04 23:00 ` Oliver Upton
2024-06-04 23:36 ` Sean Christopherson
2024-05-29 18:05 ` [PATCH v4 7/7] KVM: selftests: Add multi-gen LRU aging to access_tracking_perf_test James Houghton
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