Re: [RFC] cputime: Introduce option to force full dynticks accounting on NOHZ & NOHZ_IDLE CPUs

[Sean Christopherson]

On Mon, Feb 19, 2024, Nicolas Saenz Julienne wrote:
> Under certain extreme conditions, the tick-based cputime accounting may
> produce inaccurate data. For instance, guest CPU usage is sensitive to
> interrupts firing right before the tick's expiration. This forces the
> guest into kernel context, and has that time slice wrongly accounted as
> system time. This issue is exacerbated if the interrupt source is in
> sync with the tick, significantly skewing usage metrics towards system
> time.

..

> NOTE: This wasn't tested in depth, and it's mostly intended to highlight
> the issue we're trying to solve. Also ccing KVM folks, since it's
> relevant to guest CPU usage accounting.

How bad is the synchronization issue on upstream kernels?  We tried to address
that in commit 160457140187 ("KVM: x86: Defer vtime accounting 'til after IRQ handling").

I don't expect it to be foolproof, but it'd be good to know if there's a blatant
flaw and/or easily closed hole.