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

From: Wanpeng Li <kernellwp@gmail.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Wanpeng Li" <wanpeng.li@hotmail.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"Radim Krčmář" <rkrcmar@redhat.com>,
	"Ingo Molnar" <mingo@kernel.org>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	"Rik van Riel" <riel@redhat.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Frederic Weisbecker" <fweisbec@gmail.com>,
	"John Stultz" <john.stultz@linaro.org>
Subject: Re: [PATCH v4 1/3] KVM: fix steal clock warp during guest cpu hotplug
Date: Tue, 7 Jun 2016 19:16:30 +0800	[thread overview]
Message-ID: <CANRm+CxOJ-9WwxPOUaJOtc4dr+xgkLXWav8Zp-NgZYniA0VCOw@mail.gmail.com> (raw)
In-Reply-To: <b6cfcf78-d991-d7f3-8781-8f4e50563be9@redhat.com>

2016-06-07 18:39 GMT+08:00 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! Your help is always a great appreciated. :)

Regards,
Wanpeng Li