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

[Rik van Riel <riel@redhat.com>]

On 02/05/2015 12:50 PM, Paul E. McKenney wrote:
> 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.

Interesting, in what cases would that happen?

If context_tracking_is_enabled() we end up eventually
calling into rcu_user_enter & rcu_user_exit.

If it is not enabled, we call rcu_virt_note_context_switch.

In what cases would we need to call both?

I don't see exit-to-userspace do the things that
rcu_virt_note_context_switch does, and do not understand
why userspace is fine with that...