Re: Possible nohz-full/RCU issue in arm64 KVM

Paolo Bonzini
2021-12-17T18:12:22Z

On 12/17/21 17:45, Paul E. McKenney wrote:
> On Fri, Dec 17, 2021 at 05:34:04PM +0100, Paolo Bonzini wrote:
>> On 12/17/21 17:07, Paul E. McKenney wrote:
>>>> rcu_note_context_switch() is a point-in-time notification; it's not strictly
>>>> necessary, but it may improve performance a bit by avoiding unnecessary IPIs
>>>> from the RCU subsystem.
>>>>
>>>> There's no benefit from doing it when you're back from the guest, because at
>>>> that point the CPU is just running normal kernel code.
>>>
>>> Do scheduling-clock interrupts from guest mode have the "user" parameter
>>> set? If so, that would keep RCU happy.
>>
>> No, thread is in supervisor mode. But after every interrupt (timer tick or
>> anything), one of three things can happen:
>>
>> * KVM will go around the execution loop and invoke rcu_note_context_switch()
>> again
>>
>> * or KVM will go back to user space
>
> Here "user space" is a user process as opposed to a guest OS?

Yes, that code runs from ioctl(KVM_RUN) and the ioctl will return to the
calling process.

Paolo

>> * or the thread will be preempted
>>
>> and either will keep RCU happy as far as I understand.
>
> Regardless of the answer to my question above, yes, these will keep
> RCU happy. ;-)