Re: [PATCH v4 1/3] KVM: fix steal clock warp during guest cpu hotplug

[Paolo Bonzini <pbonzini@redhat.com>]

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