Re: A question about HTL VM-Exit handling time

[Wanpeng Li <wanpeng.li@linux.intel.com>]

Hi Paolo,
On Tue, Oct 21, 2014 at 08:39:49PM +0200, Paolo Bonzini wrote:
>
>
>On 10/16/2014 10:15 AM, Wu, Feng wrote:
>> Hi folks,
>> 
>> I run kernel build in the guest and use perf kvm to get some VM-Exit result as the following:
>> 
>> Analyze events for all VCPUs:
>> 
>>              VM-EXIT    Samples  Samples%     Time%   Min Time   Max Time         A
>> 
>>            MSR_WRITE    3613908    57.53%    18.97%        5us     1362us      9.73
>>                  HLT    1399747    22.28%    74.90%        5us   432448us     99.24
>>            CR_ACCESS     961203    15.30%     3.28%        4us      188us      6.33
>>   EXTERNAL_INTERRUPT     213821     3.40%     2.25%        4us     4089us     19.54
>>        EXCEPTION_NMI      25152     0.40%     0.12%        4us       71us      9.05
>>        EPT_MISCONFIG      20104     0.32%     0.15%        8us     5628us     13.74
>>                CPUID      19904     0.32%     0.07%        4us      220us      6.90
>>       IO_INSTRUCTION      17097     0.27%     0.20%       13us     1008us     22.08
>>    PAUSE_INSTRUCTION      10737     0.17%     0.05%        4us       53us      8.33
>>             MSR_READ         48     0.00%     0.00%        4us        8us      5.62
>> 
>> Total Samples:6281721, Total events handled time:185457820.41us.
>> 
>> I also do some other experiments with different workload in the guest, I got the same results in terms of
>> HLT VM-Exit handling time. Does anyone know why the handling time for HLT VM-Exit is so high? Appreciate
>> You help!
>
>432 ms sounds like a lot, but in general it is expected that HLT vmexits
>take a long time.  After an HLT vmexit, the VCPU will not be reentered
>until the next interrupt comes.  On hardware, the HLT instruction can
>also take many milliseconds.
>

As you know, kernel build which Feng used is a CPU bound workload,
so why 22.28% samples is halt?

Regards,
Wanpeng Li 

>If this is an SMP guest, it's possible that the maximum time is
>registered on the APs before Linux boots.  With a UP guest I would
>expect a shorter maximum time, but still longer than other vmexits.
>
>Paolo
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