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

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Date:   Wed, 11 Jul 2018 16:37:27 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>
Cc:     David Woodhouse <dwmw2@...radead.org>, peterz@...radead.org,
        mhillenb@...zon.de, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH v2] kvm/x86: Inform RCU of quiescent state when entering
 guest mode

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@...zon.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@...zon.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@...zon.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@...zon.co.uk>
> > >>>     Signed-off-by: Paul E. McKenney <paulmck@...ux.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.

							Thanx, Paul