Messages in this thread | | | Subject | Re: Possible regression in cpuacct.stats system time | From | Paolo Bonzini <> | Date | Tue, 5 Jan 2021 08:27:13 +0100 |
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On 04/01/21 22:24, Nitesh Narayan Lal wrote: > Hi, > > Last year I reported an issue of "suspicious RCU usage" [1] with the debug > kernel which was fixed with the patch: > > 87fa7f3e98 "x86/kvm: Move context tracking where it belongs" > > Recently I have come across a possible regression because of this > patch in the cpuacct.stats system time. > > With the latest upstream kernel (5.11-rc2) when we set up a VM and start > observing the system time value from cpuacct.stat then it is significantly > higher than value reported with the kernel that doesn't have the > previously mentioned patch. > > FWIU the reason behind this increase is the moving of guest_exit_irqoff() > to its proper location (near vmexit). This leads to the accounting > of instructions that were previously accounted into the guest context as a > part of the system time. > > IMO this should be an expected behavior after the previously mentioned > change. Is that a right conclusion or I am missing something here?
Yes it's expected and I think it's more precise, since this is host overhead rather than guest operation .
> Another question that I have is about the patch > > d7a08882a0 "KVM: x86: Unconditionally enable irqs in guest context" > > considering we are enabling irqs early now in the code path, do we still > need this patch?
No, we don't. Since the code is a bit simpler without it, feel free to send a revert.
Thanks,
Paolo
> > [1] https://lore.kernel.org/lkml/ece36eb1-253a-8ec6-c183-309c10bb35d5@redhat.com/ > > -- > Thanks > Nitesh >
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