Re: [PATCH 6/6] kvm,rcu,nohz: use RCU extended quiescent state when running KVM guest

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

On Tue, Feb 10, 2015 at 11:59:09AM -0800, Andy Lutomirski wrote:
> On 02/10/2015 06:41 AM, riel@redhat.com wrote:
> >From: Rik van Riel <riel@redhat.com>
> >
> >The host kernel is not doing anything while the CPU is executing
> >a KVM guest VCPU, so it can be marked as being in an extended
> >quiescent state, identical to that used when running user space
> >code.
> >
> >The only exception to that rule is when the host handles an
> >interrupt, which is already handled by the irq code, which
> >calls rcu_irq_enter and rcu_irq_exit.
> >
> >The guest_enter and guest_exit functions already switch vtime
> >accounting independent of context tracking. Leave those calls
> >where they are, instead of moving them into the context tracking
> >code.
> >
> >Signed-off-by: Rik van Riel <riel@redhat.com>
> >---
> >  include/linux/context_tracking.h       | 6 ++++++
> >  include/linux/context_tracking_state.h | 1 +
> >  include/linux/kvm_host.h               | 3 ++-
> >  3 files changed, 9 insertions(+), 1 deletion(-)
> >
> >diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
> >index 954253283709..b65fd1420e53 100644
> >--- a/include/linux/context_tracking.h
> >+++ b/include/linux/context_tracking.h
> >@@ -80,10 +80,16 @@ static inline void guest_enter(void)
> >  		vtime_guest_enter(current);
> >  	else
> >  		current->flags |= PF_VCPU;
> >+
> >+	if (context_tracking_is_enabled())
> >+		context_tracking_enter(IN_GUEST);
> 
> Why the if statement?
> 
> Also, have you checked how much this hurts guest lightweight
> entry/exit latency?  Context tracking is shockingly expensive for
> reasons I don't fully understand, but hopefully most of it is the
> vtime stuff.  (Context tracking is *so* expensive that I almost
> think we should set the performance taint flag if we enable it,
> assuming that flag ended up getting merged.  Also, we should make
> context tracking faster.)

It turns out that context_tracking_is_enabled() is a static inline
that uses a static_key, so the overhead should be minimal on platforms
having a full implementation of static keys.

							Thanx, Paul