Possible nohz-full/RCU issue in arm64 KVM

[Paul E. McKenney]

On Fri, Dec 17, 2021 at 06:23:32PM +0100, Paolo Bonzini wrote:
> On 12/17/21 18:12, Paul E. McKenney wrote:
> > On Fri, Dec 17, 2021 at 06:02:23PM +0100, Paolo Bonzini wrote:
> > > 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.
> > 
> > Intriguing.  A user process within the guest OS or a user process outside
> > of any guest OS, that is, within the host?
> 
> A user process on the host.  The guest vCPU is nothing special: it's just a
> user thread that occasionally lets the guest run by invoking the KVM_RUN
> ioctl.  Hopefully, KVM_RUN will be where that user thread will spend most of
> the time so the guest runs at full steam.  KVM_RUN is the place where you
> have the code that Nicolas and Mark were discussing.
> 
> From the point of view of the kernel however the thread is always in kernel
> mode when it runs the guest, because any interrupt will be recognized while
> still in the ioctl.
> 
> (I'll add that from the point of view of the scheduler, there's no
> difference between a CPU-bound guest and a "normal" CPU-bound process on the
> host, e.g. wasting time with "for(;;)" or calculating digits of PI is the
> same no matter if you're doing it in the guest or in the host. Likewise for
> I/O-bound guests; e.g. doing "hlt" or "wfi" constantly in the guest looks
> exactly the same to the scheduler as a process that spends its time in the
> poll() system call).

Thank you for the explanation!

							Thanx, Paul