Messages in this thread | | | Date | Fri, 17 Dec 2021 18:02:23 +0100 | Subject | Re: Possible nohz-full/RCU issue in arm64 KVM | From | Paolo Bonzini <> |
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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. ;-)
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