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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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?

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index b0c82f70afa7..0046aa70205a 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7458,9 +7458,7 @@ 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();
 
 	/*
 	 * Do this here before restoring debug registers on the host.  And
diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
index d05609ad329d..8d2a9d3073ad 100644
--- a/include/linux/context_tracking.h
+++ b/include/linux/context_tracking.h
@@ -118,12 +118,12 @@ static inline void guest_enter_irqoff(void)
 	 * one time slice). Lets treat guest mode as quiescent state, just like
 	 * we do with user-mode execution.
 	 */
-	if (!context_tracking_cpu_is_enabled())
-		rcu_virt_note_context_switch(smp_processor_id());
+	rcu_kvm_enter();
 }
 
 static inline void guest_exit_irqoff(void)
 {
+	rcu_kvm_exit();
 	if (context_tracking_is_enabled())
 		__context_tracking_exit(CONTEXT_GUEST);
 
@@ -143,12 +143,13 @@ static inline void guest_enter_irqoff(void)
 	 */
 	vtime_account_system(current);
 	current->flags |= PF_VCPU;
-	rcu_virt_note_context_switch(smp_processor_id());
+	rcu_kvm_enter();
 }
 
 static inline void guest_exit_irqoff(void)
 {
 	/* Flush the guest cputime we spent on the guest */
+	rcu_kvm_exit();
 	vtime_account_system(current);
 	current->flags &= ~PF_VCPU;
 }
diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index 4b2d691e453f..a7aa5b3cfb81 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -81,7 +81,6 @@ static inline int rcu_needs_cpu(u64 basemono, u64 *nextevt)
  * Take advantage of the fact that there is only one CPU, which
  * allows us to ignore virtualization-based context switches.
  */
-static inline void rcu_virt_note_context_switch(int cpu) { }
 static inline void rcu_cpu_stall_reset(void) { }
 static inline void rcu_idle_enter(void) { }
 static inline void rcu_idle_exit(void) { }
diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index 48ce58b53ece..62b61e579bb4 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -34,17 +34,6 @@ void rcu_softirq_qs(void);
 void rcu_note_context_switch(bool preempt);
 int rcu_needs_cpu(u64 basem, u64 *nextevt);
 void rcu_cpu_stall_reset(void);
-
-/*
- * Note a virtualization-based context switch.  This is simply a
- * wrapper around rcu_note_context_switch(), which allows TINY_RCU
- * to save a few bytes. The caller must have disabled interrupts.
- */
-static inline void rcu_virt_note_context_switch(int cpu)
-{
-	rcu_note_context_switch(false);
-}
-
 void synchronize_rcu_expedited(void);
 void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func);