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

On Fri, Dec 17, 2021 at 06:02:23PM +0100, Paolo Bonzini wrote:

On 12/17/21 17:45, Paul E. McKenney wrote:

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On Fri, Dec 17, 2021 at 05:34:04PM +0100, Paolo Bonzini wrote:

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On 12/17/21 17:07, Paul E. McKenney wrote:

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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.

Intriguing.  A user process within the guest OS or a user process outside
of any guest OS, that is, within the host?

							Thanx, Paul

Paolo

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* 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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