Re: [PATCH v2] kvm/x86: Inform RCU of quiescent state when entering guest mode

Paul E. McKenney
2018-07-12T02:13:48Z

On Wed, Jul 11, 2018 at 04:37:27PM -0700, Paul E. McKenney wrote:
> On Wed, Jul 11, 2018 at 02:32:59PM -0700, Paul E. McKenney wrote:
> > On Wed, Jul 11, 2018 at 11:11:19PM +0200, Christian Borntraeger wrote:
> > >
> > >
> > > On 07/11/2018 10:27 PM, Paul E. McKenney wrote:
> > > > On Wed, Jul 11, 2018 at 08:39:36PM +0200, Christian Borntraeger wrote:
> > > >>
> > > >>
> > > >> On 07/11/2018 08:36 PM, Paul E. McKenney wrote:
> > > >>> On Wed, Jul 11, 2018 at 11:20:53AM -0700, Paul E. McKenney wrote:
> > > >>>> On Wed, Jul 11, 2018 at 07:01:01PM +0100, David Woodhouse wrote:
> > > >>>>> From: David Woodhouse <dwmw@amazon.co.uk>
> > > >>>>>
> > > >>>>> RCU can spend long periods of time waiting for a CPU which is actually in
> > > >>>>> KVM guest mode, entirely pointlessly. Treat it like the idle and userspace
> > > >>>>> modes, and don't wait for it.
> > > >>>>>
> > > >>>>> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> > > >>>>
> > > >>>> And idiot here forgot about some of the debugging code in RCU's dyntick-idle
> > > >>>> code. I will reply with a fixed patch.
> > > >>>>
> > > >>>> The code below works just fine as long as you don't enable CONFIG_RCU_EQS_DEBUG,
> > > >>>> so should be OK for testing, just not for mainline.
> > > >>>
> > > >>> And here is the updated code that allegedly avoids splatting when run with
> > > >>> CONFIG_RCU_EQS_DEBUG.
> > > >>>
> > > >>> Thoughts?
> > > >>>
> > > >>> Thanx, Paul
> > > >>>
> > > >>> ------------------------------------------------------------------------
> > > >>>
> > > >>> commit 12cd59e49cf734f907f44b696e2c6e4b46a291c3
> > > >>> Author: David Woodhouse <dwmw@amazon.co.uk>
> > > >>> Date: Wed Jul 11 19:01:01 2018 +0100
> > > >>>
> > > >>> kvm/x86: Inform RCU of quiescent state when entering guest mode
> > > >>>
> > > >>> RCU can spend long periods of time waiting for a CPU which is actually in
> > > >>> KVM guest mode, entirely pointlessly. Treat it like the idle and userspace
> > > >>> modes, and don't wait for it.
> > > >>>
> > > >>> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> > > >>> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > > >>> [ paulmck: Adjust to avoid bad advice I gave to dwmw, avoid WARN_ON()s. ]
> > > >>>
> > > >>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > >>> index 0046aa70205a..b0c82f70afa7 100644
> > > >>> --- a/arch/x86/kvm/x86.c
> > > >>> +++ b/arch/x86/kvm/x86.c
> > > >>> @@ -7458,7 +7458,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> > > >>> vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD;
> > > >>> }
> > > >>>
> > > >>> + rcu_kvm_enter();
> > > >>> kvm_x86_ops->run(vcpu);
> > > >>> + rcu_kvm_exit();
> > > >>
> > > >> As indicated in my other mail. This is supposed to be handled in the guest_enter|exit_ calls around
> > > >> the run function. This would also handle other architectures. So if the guest_enter_irqoff code is
> > > >> not good enough, we should rather fix that instead of adding another rcu hint.
> > > >
> > > > Something like this, on top of the earlier patch? I am not at all
> > > > confident of this patch because there might be other entry/exit
> > > > paths I am missing. Plus there might be RCU uses on the arch-specific
> > > > patch to and from the guest OS.
> > > >
> > > > Thoughts?
> > > >
> > >
> > > If you instrment guest_enter/exit, you should cover all cases and all architectures as far
> > > as I can tell. FWIW, we did this rcu_note thing back then actually handling this particular
> > > case of long running guests blocking rcu for many seconds. And I am pretty sure that
> > > this did help back then.
> >
> > And my second patch on the email you replied to replaced the only call
> > to rcu_virt_note_context_switch(). So maybe it covers what it needs to,
> > but yes, there might well be things I missed. Let's see what David
> > comes up with.
> >
> > What changed was RCU's reactions to longish grace periods. It used to
> > be very aggressive about forcing the scheduler to do otherwise-unneeded
> > context switches, which became a problem somewhere between v4.9 and v4.15.
> > I therefore reduced the number of such context switches, which in turn
> > caused KVM to tell RCU about quiescent states way too infrequently.
> >
> > The advantage of the rcu_kvm_enter()/rcu_kvm_exit() approach is that
> > it tells RCU of an extended duration in the guest, which means that
> > RCU can ignore the corresponding CPU, which in turn allows the guest
> > to proceed without any RCU-induced interruptions.
> >
> > Does that make sense, or am I missing something? I freely admit to
> > much ignorance of both kvm and s390! ;-)
>
> But I am getting some rcutorture near misses on the commit that
> introduces rcu_kvm_enter() and rcu_kvm_exit() to the x86 arch-specific
> vcpu_enter_guest() function. These near misses occur when running
> rcutorture scenarios TREE01 and TREE03, and in my -rcu tree rather
> than the v4.15 version of this patch.
>
> Given that I am making pervasive changes to the way that RCU works,
> it might well be that this commit is an innocent bystander. I will
> run tests overnight and let you know what comes up.

And thus far, the verdict is "intermittent". I am starting a new
series with CONFIG_RCU_EQS_DEBUG=y, which should detect any mismatched
rcu_kvm_enter()/rcu_kvm_exit() pairs.

Thanx, Paul