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 r...@redhat.com:
>>>> From: Rik van Riel <r...@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 <r...@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...
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