Re: [RESEND 2][PATCH 4/4] Modify KVM to update guest time accounting.

[Laurent Vivier <Laurent.Vivier@bull.net>]

No more comments: I agree.

We can move the "&= ~PF_VCPU" to kvm_guest_exit() and remove it from 
account_system_time(). Moreover it will simplify the code for s390...

Regards,
Laurent

Avi Kivity wrote:
> Laurent Vivier wrote:
> > Avi Kivity wrote:
> >  
> > > Laurent Vivier wrote:
> > >
> > >  
> > >
> > >    
> > > > > But if we didn't get an interrupt in that time?
> > > > >
> > > > > We can clear it a bit later, after local_irq_enable() in
> > > > > __vcpu_run(). However we need a nop instruction first because "sti"
> > > > > keeps interrupts
> > > > > disabled for one more instruction.
> > > > >             
> > > > IMHO, I think it is better to let kvm_guest_exit() empty (you can
> > > > remove it, if
> > > > you want):
> > > >
> > > > 1st case:
> > > > - unset PF_VCPU in kvm_guest_exit(), all the tick is always for system
> > > > time.
> > > > Guest time is always 0.
> > > >
> > > > 1st case and half:
> > > >
> > > > - like 1st case but we move kvm_guest_exit() as you propose and the
> > > > reason of
> > > > the interrupt is the tick interrupt. The tick is for guest time only.
> > > > I think
> > > > the probability is very low.
> > > >         
> > > If the guest is executing for 10% of the time, the probability is
> > > exactly 10%, no?
> > >     
> >
> > I think you know that better than me.
> >
> > But is there homogeneity in probability ?
> >   
>
> It's exactly the same issue as with systime and usertime.  The interrupt 
> samples the program counter at various points at a fairly low frequency 
> (milliseconds) while syscalls last a few dozens of microseconds.  
> Probability makes it average out correctly in the end.
>
> [Ingo, what about dyntick?  suppose you have just one process that calls 
> read() from /dev/zero repeatedly.  There'd be very few (or no) 
> interrupts -- what happens to accounting accuracy?]
>
> > I mean, if the guest has a lot I/O, it is interrupted by them and the
> > probability to be interrupted by a tick is lower than the time passed 
> > in the VCPU ?
> >   
>
> Suppose the time to service the I/O is exactly equal to the amount 
> running in guest mode.   Then the probability of the interrupt happening 
> in guest mode is equal to it happening outside guest mode and you'd get 
> 50% guest, 50% system/user, which is what you want.
>
>
> >  
> > > > 2nd case:
> > > > - don't unset PF_VCPU in kvm_guest_exit(), all the tick is for guest
> > > > time.
> > > >         
> > > But then even execution in ->handle_exit() is accounted as guest time,
> > > which is wrong.
> > >     
> >
> > System time and User time are wrong too as the tick is accounted to 
> > the side
> > where it appears, even if CPU has executed code from the other side in a
> > sub-part of the tick. It's not a good argument.
> >   
>
> It's at least consistent... the same errors for everyone, so it averages 
> out in the end.

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