From: Paolo Bonzini <pbonzini@redhat.com>
To: Nitesh Narayan Lal <nitesh@redhat.com>,
linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
tglx@linutronix.de, alexandre.chartre@oracle.com,
peterz@infradead.org, w90p710@gmail.com,
sean.j.christopherson@intel.com, vkuznets@redhat.com
Subject: Re: Possible regression in cpuacct.stats system time
Date: Tue, 5 Jan 2021 08:27:13 +0100 [thread overview]
Message-ID: <49664b8e-8b14-eb5c-f25c-da604b7c077b@redhat.com> (raw)
In-Reply-To: <12a1b9d4-8534-e23a-6bbd-736474928e6b@redhat.com>
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
>
prev parent reply other threads:[~2021-01-05 7:28 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-04 21:24 Possible regression in cpuacct.stats system time Nitesh Narayan Lal
2021-01-05 7:27 ` Paolo Bonzini [this message]
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