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

Paul E. McKenney
2018-07-11T23:45:09Z

On Wed, Jul 11, 2018 at 11:39:10PM +0200, Christian Borntraeger wrote:
>
>
> On 07/11/2018 11:32 PM, 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.
>
> You talk about
> commit bcbfdd01dce5556a952fae84ef16fd0f12525e7b
> rcu: Make non-preemptive schedule be Tasks RCU quiescent state
>
> correct? In fact, then whatever (properly sent) patch comes up should contain
> a fixes tag.

Not that one, but this one is at least part of the "team":

28053bc72c0e5 ("rcu: Add long-term CPU kicking"). I might need to use
"git bisect" to find the most relevant commit... :-/

> > 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! ;-)
>
> WIth that explanation it makes perfect sense to replace
> rcu_virt_note_context_switch with rcu_kvm_enter/exit from an rcu performance
> perspective. I assume that rcu_kvm_enter is not much slower than
> rcu_virt_note_context_switch? Because we do call it on every guest entry/exit
> which we might have plenty for ping pong I/O workload.

But is there any way for a guest OS to sneak back out to the hypervisor
without executing one of the rcu_kvm_exit() calls? If there is, RCU
is broken.

Thanx, Paul