Re: vtime accounting

["Radim Krčmář" <rkrcmar@redhat.com>]

2017-03-14 21:27+0100, Radim Krčmář:
> 2017-03-14 19:39+0100, Christoffer Dall:
>> On Tue, Mar 14, 2017 at 05:58:59PM +0100, Radim Krčmář wrote:
>>> 2017-03-14 09:26+0100, Christoffer Dall:
>>> > 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?
>>> 
>>> Probably, I don't understand jiffies accounting too well and didn't see
>>> anything that would bump the jiffies in or before guest_exit_irqoff().
>> 
>> As far as I understand, from my very very short time of looking at the
>> timer code, jiffies are updated on every tick, which can be cause by a
>> number of events, including *any* interrupt handler (coming from idle
>> state), soft timers, timer interrupts, and possibly other things.
> 
> Yes, I was thinking that entering/exiting user mode should trigger it as
> well, in order to correctly account for time spent there, but couldn't
> find it ...
> 
> The case I was wondering about is if the kernel spent e.g. 10 jiffies in
> guest mode and then exited on mmio -- no interrupt in the host, and
> guest_exit_irqoff() would flip accouting would over system time.
> Can those 10 jiffies get accounted to system (not guest) time?
> 
> Accounting 1 jiffy wrong is normal as we expect that the distribution of
> guest/kernel time in the jiffy is going to be approximated over a longer
> sampling period, but if we could account multiple jiffies wrong, this
> expectation is very hard to defend.

IIUC, other CPU will bump jiffies of running tickless VCPUs, so this
works fine.