linux-kernel - Re: [PATCH mm-unstable v1 5/5] mm: multi-gen LRU: use mmu_notifier_test_clear

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Date:   Thu, 23 Feb 2023 13:09:33 -0700
From:   Yu Zhao <yuzhao@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Michael Larabel <michael@...haellarabel.com>,
        kvmarm@...ts.linux.dev, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
        linux-mm@...gle.com
Subject: Re: [PATCH mm-unstable v1 5/5] mm: multi-gen LRU: use mmu_notifier_test_clear_young()

On Thu, Feb 23, 2023 at 12:58 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Thu, Feb 23, 2023, Yu Zhao wrote:
> > On Thu, Feb 23, 2023 at 12:11 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > >
> > > On Thu, Feb 23, 2023, Yu Zhao wrote:
> > > > > As alluded to in patch 1, unless batching the walks even if KVM does _not_ support
> > > > > a lockless walk is somehow _worse_ than using the existing mmu_notifier_clear_flush_young(),
> > > > > I think batching the calls should be conditional only on LRU_GEN_SPTE_WALK.  Or
> > > > > if we want to avoid batching when there are no mmu_notifier listeners, probe
> > > > > mmu_notifiers.  But don't call into KVM directly.
> > > >
> > > > I'm not sure I fully understand. Let's present the problem on the MM
> > > > side: assuming KVM supports lockless walks, batching can still be
> > > > worse (very unlikely), because GFNs can exhibit no memory locality at
> > > > all. So this option allows userspace to disable batching.
> > >
> > > I'm asking the opposite.  Is there a scenario where batching+lock is worse than
> > > !batching+lock?  If not, then don't make batching depend on lockless walks.
> >
> > Yes, absolutely. batching+lock means we take/release mmu_lock for
> > every single PTE in the entire VA space -- each small batch contains
> > 64 PTEs but the entire batch is the whole KVM.
>
> Who is "we"?

Oops -- shouldn't have used "we".

> I don't see anything in the kernel that triggers walking the whole
> VMA, e.g. lru_gen_look_around() limits the walk to a single PMD.  I feel like I'm
> missing something...

walk_mm() -> walk_pud_range() -> walk_pmd_range() -> walk_pte_range()
-> test_spte_young() -> mmu_notifier_test_clear_young().

MGLRU takes two passes: during the first pass, it sweeps entire VA
space on each MM (per MM/KVM); during the second pass, it uses the rmap on each
folio (per folio). The look around exploits the (spatial) locality in
the second pass, to get the best out of the expensive per folio rmap
walk.

(The first pass can't handle shared mappings; the second pass can.)