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

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Message-ID: <Z4U_FvvdSBXrzENW@google.com>
Date: Mon, 13 Jan 2025 08:28:06 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Peter Xu <peterx@...hat.com>, Maxim Levitsky <mlevitsk@...hat.com>
Subject: Re: [PATCH 3/5] KVM: Conditionally reschedule when resetting the
 dirty ring

On Mon, Jan 13, 2025, Yan Zhao wrote:
> On Fri, Jan 10, 2025 at 05:04:07PM -0800, Sean Christopherson wrote:
> > diff --git a/virt/kvm/dirty_ring.c b/virt/kvm/dirty_ring.c
> > index a81ad17d5eef..37eb2b7142bd 100644
> > --- a/virt/kvm/dirty_ring.c
> > +++ b/virt/kvm/dirty_ring.c
> > @@ -133,6 +133,16 @@ 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();
> > +
> Will cond_resched() per entry be too frequent?

No, if it is too frequent, KVM has other problems.  cond_resched() only takes a
handful of cycles when no work needs to be done, and on PREEMPTION=y kernels,
dropping mmu_lock in kvm_reset_dirty_gfn() already includes a NEED_RESCHED check.

> Could we combine the cond_resched() per ring? e.g.
> 
> if (count >= ring->soft_limit)
> 	cond_resched();
> 
> or simply
> while (count < ring->size) {
> 	...
> }

I don't think I have any objections to bounding the reset at ring->size?  I
assumed the unbounded walk was deliberate, e.g. to let userspace reset entries
in a separate thread, but looking at the QEMU code, that doesn't appear to be
the case.

However, IMO that's an orthogonal discussion.  I think KVM should still check for
NEED_RESCHED after processing each entry regardless of how the loop is bounded.
E.g. write-protecting 65536 GFNs is definitely going to have measurable latency.