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

Paul E. McKenney
2018-07-11T20:25:45Z

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);