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 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();
 
 	/*
 	 * Do this here before restoring debug registers on the host.  And
diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index 7fa4fb9e899e..4b2d691e453f 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -85,6 +85,8 @@ 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) { }
+static inline void rcu_kvm_enter(void) { }
+static inline void rcu_kvm_exit(void) { }
 static inline void rcu_irq_enter(void) { }
 static inline void rcu_irq_exit_irqson(void) { }
 static inline void rcu_irq_enter_irqson(void) { }
diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index 7f83179177d1..48ce58b53ece 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -55,6 +55,8 @@ void cond_synchronize_rcu(unsigned long oldstate);
 
 void rcu_idle_enter(void);
 void rcu_idle_exit(void);
+void rcu_kvm_enter(void);
+void rcu_kvm_exit(void);
 void rcu_irq_enter(void);
 void rcu_irq_exit(void);
 void rcu_irq_enter_irqson(void);
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 765c81dd675e..0c0672faa6d1 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -583,6 +583,24 @@ void rcu_idle_enter(void)
 	rcu_eqs_enter(false);
 }
 
+/**
+ * rcu_kvm_enter - inform RCU that current CPU is entering a guest OS
+ *
+ * Enter guest-OS mode, in other words, -leave- the mode in which RCU
+ * read-side critical sections can occur.  (Though RCU read-side critical
+ * sections can occur in irq handlers from guest OSes, a possibility
+ * handled by irq_enter() and irq_exit().)
+ *
+ * If you add or remove a call to rcu_kvm_enter(), be sure to test with
+ * CONFIG_RCU_EQS_DEBUG=y.
+ */
+void rcu_kvm_enter(void)
+{
+	lockdep_assert_irqs_disabled();
+	rcu_eqs_enter(true);
+}
+EXPORT_SYMBOL_GPL(rcu_kvm_enter);
+
 #ifdef CONFIG_NO_HZ_FULL
 /**
  * rcu_user_enter - inform RCU that we are resuming userspace.
@@ -747,6 +765,22 @@ void rcu_idle_exit(void)
 	local_irq_restore(flags);
 }
 
+/**
+ * rcu_kvm_exit - inform RCU that current CPU is leaving a guest OS
+ *
+ * Exit guest-OS mode, in other words, -enter- the mode in which RCU
+ * read-side critical sections can occur.
+ *
+ * If you add or remove a call to rcu_kvm_exit(), be sure to test with
+ * CONFIG_RCU_EQS_DEBUG=y.
+ */
+void rcu_kvm_exit(void)
+{
+	lockdep_assert_irqs_disabled();
+	rcu_eqs_exit(true);
+}
+EXPORT_SYMBOL_GPL(rcu_kvm_exit);
+
 #ifdef CONFIG_NO_HZ_FULL
 /**
  * rcu_user_exit - inform RCU that we are exiting userspace.