Re: vtime accounting

Rik van Riel
2017-03-24T14:55:52Z

On Tue, 2017-03-14 at 09:26 +0100, Christoffer Dall wrote:
> On Mon, Mar 13, 2017 at 06:28:16PM +0100, Radim Krčmář wrote:
> > 2017-03-08 02:57-0800, Christoffer Dall:
> > > Hi Paolo,
> > >
> > > I'm looking at improving KVM/ARM a bit by calling
> > > guest_exit_irqoff
> > > before enabling interrupts when coming back from the guest.
> > >
> > > Unfortunately, this appears to mess up my view of CPU usage using
> > > something like htop on the host, because it appears all time is
> > > spent
> > > inside the kernel.
> > >
> > > From my analysis, I think this is because we never handle any
> > > interrupts
> > > before enabling interrupts, where the x86 code does its
> > > handle_external_intr, and the result on ARM is that we never
> > > increment
> > > jiffies before doing the vtime accounting.
> >
> > (Hm, the counting might be broken on nohz_full then.)
> >
>
> Don't you still have a scheduler tick even with nohz_full and
> something
> that will eventually update jiffies then?
>

With nohz_full, the housekeeping CPU will get a
timer interrupt every single jiffy, and the
jiffies variable will get updated HZ times a
second.