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SubjectRe: [PATCH v4 1/3] KVM: fix steal clock warp during guest cpu hotplug
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On 07/06/2016 09:59, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@hotmail.com>
>
> I observed that sometimes st is 100% instantaneous, then idle is 100%
> even if there is a cpu hog on the guest cpu after the cpu hotplug comes
> back(N.B. this can not always be readily reproduced). I add trace to
> capture it as below:
>
> cpuhp/1-12 [001] d.h1 167.461657: account_process_tick: steal = 1291385514, prev_steal_time = 0
> cpuhp/1-12 [001] d.h1 167.461659: account_process_tick: steal_jiffies = 1291
> <idle>-0 [001] d.h1 167.462663: account_process_tick: steal = 18732255, prev_steal_time = 1291000000
> <idle>-0 [001] d.h1 167.462664: account_process_tick: steal_jiffies = 18446744072437
>
> The steal clock warp and then steal_jiffies underflow.
>
> Rik also pointed out to me:
>
> | I have seen stuff like that with live migration too, in the past
>
> The root cause of steal clock warp during hotplug is kvm_steal_time reset
> to 0 after cpu hotplug comes back which should be preexiting guest value.
> This patch fix it by don't reset kvm_steal_time during guest cpu hotplug.

Improved commit message:

Sometimes, after CPU hotplug you can observe a spike in stolen time
(100%) followed by the CPU being marked as 100% idle when it's actually
busy with a CPU hog task. The trace looks like the following:

cpuhp/1-12 [001] d.h1 167.461657: account_process_tick: steal = 1291385514, prev_steal_time = 0
cpuhp/1-12 [001] d.h1 167.461659: account_process_tick: steal_jiffies = 1291
<idle>-0 [001] d.h1 167.462663: account_process_tick: steal = 18732255, prev_steal_time = 1291000000
<idle>-0 [001] d.h1 167.462664: account_process_tick: steal_jiffies = 18446744072437

The sudden decrease of "steal" causes steal_jiffies to underflow.
The root cause is kvm_steal_time being reset to 0 after hot-plugging
back in a CPU. Instead, the preexisting value can be used, which is
what the core scheduler code expects.

John Stultz also reported a similar issue after guest S3.
------

Please also add

Cc: John Stultz <john.stultz@linaro.org>

Thanks,

Paolo

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