Re: [Question] About the behavior of HLT in VMX guest mode

["Longpeng (Mike)" <longpeng2@huawei.com>]

On 2017/3/16 10:08, Longpeng (Mike) wrote:

> Hi, Radim,
> 
> On 2017/3/16 1:32, Radim Krčmář wrote:
> 
>> 2017-03-13 15:12+0800, Longpeng (Mike):
>>> Hi guys,
>>>
>>> I'm confusing about the behavior of HLT instruction in VMX guest mode.
>>>
>>> I set "hlt exiting" bit to 0 in VMCS, and the vcpu didn't vmexit when execute
>>> HLT as expected. However, I used powertop/cpupower on host to watch the pcpu's
>>> c-states, it seems that the pcpu didn't enter C1/C1E state during this period.
>>>
>>> I searched the Intel spec vol-3, and only found that guest MWAIT won't entering
>>> a low-power sleep state under certain conditions(ch 25.3), but not mentioned HLT.
>>>
>>> My questions are
>>> 1) Does executing HLT instruction in guest-mode won't enter C1/C1E state ?
>>
>> Do you get a different result when running HLT outside VMX?
>>
> 
> 
> Yep, I'm sure that executing HLT in host will enter C1/C1E state, but it won't
> when executing in guest.
> 
>>> 2) If it won't, then whether it would release the hardware resources shared with
>>> another hyper-thread ?
>>
> 
>> No idea.  Aren't hyperthreaded resources scheduled dynamically, so even
>> a nop-spinning VCPU won't hinder the other hyper-thread?
>>
> 
> 
> I had wrote a testcase in kvm-unit-tests, and it seems that guest-mode HLT-ed
> vcpu won't compete the hardware resources( maybe including the pipeline ) any more.
> 
> My testcase is: binding vcpu1 and vcpu2 to a core's 2 hyper-threads, and
> 
> (vcpu1)
> t1 = rdtsc();
> for (int i = 0; i < 10000000; ++i) ;
> t2 = rdtsc();
> costs = t2 - t1;
> 
> (vcpu2)
> "halt" or "while (1) ;"
> 
> The result is:
> -----------------------------------------------------------------------
> 			(vcpu2)idle=poll	(vcpu2)idle=halt
> (HLT exiting=1)
> vcpu1 costs		3800931			1900209
> 
> (HLT exiting=0)
> vcpu1 costs		3800193			1913514
> -----------------------------------------------------------------------
> 
> I found that https://www.spinics.net/lists/kvm-commits/msg00137.html had maked
> "HLT exiting" configurable, while
> http://lkml.iu.edu/hypermail/linux/kernel/1202.0/03309.html removed it due to
> redundant with CFS hardlimit.
> 
> I focus on the VM's performance. According the result, I think running HLT in
> guest-mode is better than idle=poll with HLT-exiting in *certain* scenarios.
> 


comparing "HLT exiting=0 && idle=halt" with "HLT exiting=1 && idle=poll" :)

>>> Any suggestion would be greatly appreciated, thanks!
>>
>> Mostly just more questions, sorry ...
>>
>> I'd look at temperature sensors while halting inside guests on all cores
>> to see if they really enter a power saving mode -- I expect a noticeable
>> difference from idle=poll. :)
>>
>>
> 
> 


-- 
Regards,
Longpeng(Mike)