Re: [PATCH 4/4] kvm,rcu: use RCU extended quiescent state when running KVM guest

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

On Thu, Feb 05, 2015 at 11:52:37AM -0500, Rik van Riel wrote:
> On 02/05/2015 11:44 AM, Christian Borntraeger wrote:
> > Am 05.02.2015 um 17:35 schrieb riel@redhat.com:
> >> 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, so 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       | 8 +++++++-
> >>  include/linux/context_tracking_state.h | 1 +
> >>  2 files changed, 8 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
> >> index bd9f000fc98d..a5d3bb44b897 100644
> >> --- a/include/linux/context_tracking.h
> >> +++ b/include/linux/context_tracking.h
> >> @@ -43,7 +43,7 @@ static inline enum ctx_state exception_enter(void)
> >>  static inline void exception_exit(enum ctx_state prev_ctx)
> >>  {
> >>  	if (context_tracking_is_enabled()) {
> >> -		if (prev_ctx == IN_USER)
> >> +		if (prev_ctx == IN_USER || prev_ctx == IN_GUEST)
> >>  			context_tracking_user_enter(prev_ctx);
> >>  	}
> >>  }
> >> @@ -78,6 +78,9 @@ static inline void guest_enter(void)
> >>  		vtime_guest_enter(current);
> >>  	else
> >>  		current->flags |= PF_VCPU;
> >> +
> >> +	if (context_tracking_is_enabled())
> >> +		context_tracking_user_enter(IN_GUEST);
> >>  }
> > 
> > 
> > Couldnt we make
> >     rcu_virt_note_context_switch(smp_processor_id());
> > conditional in include/linux/kvm_host.h (kvm_guest_enter)
> > 
> > e.g. something like
> >     if (!context_tracking_is_enabled())
> > 	    rcu_virt_note_context_switch(smp_processor_id());
> 
> Possibly. I considered the same, but I do not know whether
> or not just rcu_user_enter / rcu_user_exit is enough.
> 
> I could certainly try it out and see whether anything
> explodes, but I am not convinced that is careful enough
> when it comes to handling RCU code...
> 
> Paul? :)

That can fail for some odd combinations of Kconfig and boot parameters.
As I understand it, you instead want the following:

	if (!tick_nohz_full_cpu(smp_processor_id()))
		rcu_virt_note_context_switch(smp_processor_id());

Frederic might know of a better approach.

							Thanx, Paul