linux-kernel - Re: [PATCH] KVM: x86: switch hugepage recovery thread to vhost

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Message-ID: <ZzYZZ4MgMhavYDM2@google.com>
Date: Thu, 14 Nov 2024 07:38:15 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	michael.christie@...cle.com, Tejun Heo <tj@...nel.org>, 
	Luca Boccassi <bluca@...ian.org>
Subject: Re: [PATCH] KVM: x86: switch hugepage recovery thread to vhost_task

On Thu, Nov 14, 2024, Paolo Bonzini wrote:
> On 11/14/24 00:56, Sean Christopherson wrote:
> > > +static bool kvm_nx_huge_page_recovery_worker(void *data)
> > > +{
> > > +	struct kvm *kvm = data;
> > >   	long remaining_time;
> > > -	while (true) {
> > > -		start_time = get_jiffies_64();
> > > -		remaining_time = get_nx_huge_page_recovery_timeout(start_time);
> > > +	if (kvm->arch.nx_huge_page_next == NX_HUGE_PAGE_DISABLED)
> > > +		return false;
> > 
> > The "next" concept is broken.  Once KVM sees NX_HUGE_PAGE_DISABLED for a given VM,
> > KVM will never re-evaluate nx_huge_page_next.  Similarly, if the recovery period
> > and/or ratio changes, KVM won't recompute the "next" time until the current timeout
> > has expired.
> > 
> > I fiddled around with various ideas, but I don't see a better solution that something
> > along the lines of KVM's request system, e.g. set a bool to indicate the params
> > changed, and sprinkle smp_{r,w}mb() barriers to ensure the vhost task sees the
> > new params.
> 
> "next" is broken, but there is a much better way to fix it.  You just
> track the *last* time that the recovery ran.  This is also better
> behaved when you flip recovery back and forth to disabled and back
> to enabled: if your recovery period is 1 minute, it will run the
> next recovery after 1 minute independent of how many times you flipped
> the parameter.

Heh, I my brain was trying to get there last night, but I couldn't quite piece
things together.

Reviewed-by: Sean Christopherson <seanjc@...gle.com>