Re: [PATCH v6 1/3] KVM: fix steal clock warp during guest cpu hotplug

[Wanpeng Li]

2016-06-13 18:44 GMT+08:00 Paolo Bonzini <pbonzini@xxxxxxxxxx>:
>
>
> On 13/06/2016 12:32, Wanpeng Li wrote:
>> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>>
>> 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.
>>
>> Suggested-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>> Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
>> Cc: Rik van Riel <riel@xxxxxxxxxx>
>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
>> Cc: John Stultz <john.stultz@xxxxxxxxxx>
>> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>> ---
>>  arch/x86/kernel/kvm.c | 2 --
>>  1 file changed, 2 deletions(-)
>>
>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>> index eea2a6f..1ef5e48 100644
>> --- a/arch/x86/kernel/kvm.c
>> +++ b/arch/x86/kernel/kvm.c
>> @@ -301,8 +301,6 @@ static void kvm_register_steal_time(void)
>>       if (!has_steal_clock)
>>               return;
>>
>> -     memset(st, 0, sizeof(*st));
>> -
>>       wrmsrl(MSR_KVM_STEAL_TIME, (slow_virt_to_phys(st) | KVM_MSR_ENABLED));
>>       pr_info("kvm-stealtime: cpu %d, msr %llx\n",
>>               cpu, (unsigned long long) slow_virt_to_phys(st));
>>
>
> Because there's no cover letter, I guess I have to ack each patch
> independently.
>
> Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

Thanks for your and Rik's review, actually there is a cover letter for
this version, it seems that it just send to ML and forgot to Cc
maintainers/reviewers.

Regards,
Wanpeng Li