linux-kernel - Re: [PATCH v2 3/5] KVM: Conditionally reschedule when resetting the dirty ring

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Message-ID: <CADrL8HUaofWTcV7X5b1AXEud03bC+gfZiecyFpux9m1tC25hOg@mail.gmail.com>
Date: Tue, 13 May 2025 15:27:09 -0700
From: James Houghton <jthoughton@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Peter Xu <peterx@...hat.com>, Yan Zhao <yan.y.zhao@...el.com>, 
	Maxim Levitsky <mlevitsk@...hat.com>
Subject: Re: [PATCH v2 3/5] KVM: Conditionally reschedule when resetting the
 dirty ring

On Tue, May 13, 2025 at 7:13 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Mon, May 12, 2025, James Houghton wrote:
> > On Thu, May 8, 2025 at 7:11 AM Sean Christopherson <seanjc@...gle.com> wrote:
> > > ---
> > >  virt/kvm/dirty_ring.c | 10 ++++++++++
> > >  1 file changed, 10 insertions(+)
> > >
> > > diff --git a/virt/kvm/dirty_ring.c b/virt/kvm/dirty_ring.c
> > > index e844e869e8c7..97cca0c02fd1 100644
> > > --- a/virt/kvm/dirty_ring.c
> > > +++ b/virt/kvm/dirty_ring.c
> > > @@ -134,6 +134,16 @@ int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring,
> > >
> > >                 ring->reset_index++;
> > >                 (*nr_entries_reset)++;
> > > +
> > > +               /*
> > > +                * While the size of each ring is fixed, it's possible for the
> > > +                * ring to be constantly re-dirtied/harvested while the reset
> > > +                * is in-progress (the hard limit exists only to guard against
> > > +                * wrapping the count into negative space).
> > > +                */
> > > +               if (!first_round)
> > > +                       cond_resched();
> >
> > Should we be dropping slots_lock here?
>
> Could we?  Yes.  Should we?  Eh.  I don't see any value in doing so, because in
> practice, it's extremely unlikely anything will be waiting on slots_lock.
>
> kvm_vm_ioctl_reset_dirty_pages() operates on all vCPUs, i.e. there won't be
> competing calls to reset other rings.  A well-behaved userspace won't be modifying
> memslots or dirty logs, and won't be toggling nx_huge_pages.
>
> That leaves kvm_vm_ioctl_set_mem_attributes(), kvm_inhibit_apic_access_page(),
> kvm_assign_ioeventfd(), snp_launch_update(), and coalesced IO/MMIO (un)registration.
> Except for snp_launch_update(), those are all brutally slow paths, e.g. require
> SRCU synchronization and/or zapping of SPTEs.  And snp_launch_update() is probably
> fairly slow too.

Okay, that makes sense.

> And dropping slots_lock only makes any sense for non-preemptible kernels, because
> preemptible kernels include an equivalent check in KVM_MMU_UNLOCK().

I'm not really sure what "equivalent check" you mean, sorry. For
preemptible kernels, we could reschedule at any time, so dropping the
slots_lock on a cond_resched() wouldn't do much, yeah. Hopefully
that's partially what you mean.

> It's also possible that dropping slots_lock in this case could be a net negative.
> I don't think it's likely, but I don't think it's any more or less likely that
> droppings slots_lock is a net positive.  Without performance data to guide us,
> it'd be little more than a guess, and I really, really don't want to set a
> precedence of dropping a mutex on cond_resched() without a very strong reason
> for doing so.

Fair enough.

Also, while we're at it, could you add a
`lockdep_assert_held(&kvm->slots_lock)` to this function? :) Not
necessarily in this patch.