linux-kernel - Re: [RESEND 2][PATCH 4/4] Modify KVM to update guest time accounting.

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Date:	Mon, 15 Oct 2007 21:45:18 +0200
From:	Laurent Vivier <Laurent.Vivier@...l.net>
To:	Avi Kivity <avi@...ranet.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RESEND 2][PATCH 4/4] Modify KVM to update guest time	accounting.

No more comments: I agree.

We can move the "&= ~PF_VCPU" to kvm_guest_exit() and remove it from 
account_system_time(). Moreover it will simplify the code for s390...

Regards,
Laurent

Avi Kivity wrote:
> Laurent Vivier wrote:
>> Avi Kivity wrote:
>>  
>>> Laurent Vivier wrote:
>>>
>>>  
>>>
>>>    
>>>>> But if we didn't get an interrupt in that time?
>>>>>
>>>>> We can clear it a bit later, after local_irq_enable() in
>>>>> __vcpu_run(). However we need a nop instruction first because "sti"
>>>>> keeps interrupts
>>>>> disabled for one more instruction.
>>>>>             
>>>> IMHO, I think it is better to let kvm_guest_exit() empty (you can
>>>> remove it, if
>>>> you want):
>>>>
>>>> 1st case:
>>>> - unset PF_VCPU in kvm_guest_exit(), all the tick is always for system
>>>> time.
>>>> Guest time is always 0.
>>>>
>>>> 1st case and half:
>>>>
>>>> - like 1st case but we move kvm_guest_exit() as you propose and the
>>>> reason of
>>>> the interrupt is the tick interrupt. The tick is for guest time only.
>>>> I think
>>>> the probability is very low.
>>>>         
>>> If the guest is executing for 10% of the time, the probability is
>>> exactly 10%, no?
>>>     
>>
>> I think you know that better than me.
>>
>> But is there homogeneity in probability ?
>>   
> 
> It's exactly the same issue as with systime and usertime.  The interrupt 
> samples the program counter at various points at a fairly low frequency 
> (milliseconds) while syscalls last a few dozens of microseconds.  
> Probability makes it average out correctly in the end.
> 
> [Ingo, what about dyntick?  suppose you have just one process that calls 
> read() from /dev/zero repeatedly.  There'd be very few (or no) 
> interrupts -- what happens to accounting accuracy?]
> 
>> I mean, if the guest has a lot I/O, it is interrupted by them and the
>> probability to be interrupted by a tick is lower than the time passed 
>> in the VCPU ?
>>   
> 
> Suppose the time to service the I/O is exactly equal to the amount 
> running in guest mode.   Then the probability of the interrupt happening 
> in guest mode is equal to it happening outside guest mode and you'd get 
> 50% guest, 50% system/user, which is what you want.
> 
> 
>>  
>>>> 2nd case:
>>>> - don't unset PF_VCPU in kvm_guest_exit(), all the tick is for guest
>>>> time.
>>>>         
>>> But then even execution in ->handle_exit() is accounted as guest time,
>>> which is wrong.
>>>     
>>
>> System time and User time are wrong too as the tick is accounted to 
>> the side
>> where it appears, even if CPU has executed code from the other side in a
>> sub-part of the tick. It's not a good argument.
>>   
> 
> It's at least consistent... the same errors for everyone, so it averages 
> out in the end.
> 

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