Messages in this thread | | | Date | Tue, 20 Feb 2024 08:18:17 -0800 | Subject | Re: [RFC] cputime: Introduce option to force full dynticks accounting on NOHZ & NOHZ_IDLE CPUs | From | Sean Christopherson <> |
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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.
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> 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.
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