Re: [kvm-devel] [PATCH/RFC 3/4]Introduce "account modifiers" mechanism

[Avi Kivity <avi@qumranet.com>]

Laurent Vivier wrote:
>>     
>>> - remove PATCH 3, and add in task_struct a "ktime vtime" where we accumulate
>>> guest time (by calling something like guest_enter() and guest_exit() from the
>>> virtualization engine), and when in account_system_time() we have cputime >
>>> vtime we substrate vtime from cputime and add vtime to user time and guest time.
>>> But doing like this we freeze in kernel/sched.c the link between system time,
>>> user time and guest time (i.e. system time = system time - vtime, user time =
>>> user time + vtime and guest time = guest time + vtime).
>>>       
>> Actually, I think we can set a per-cpu "in_guest" flag for the scheduler
>> code, which then knows to add the tick to the guest time.  That seems
>> the simplest possible solution.
>>
>> lguest or kvm would set the flag before running the guest (which is done
>> with preempt disabled or using preemption hooks), and reset it
>> afterwards.
>>
>> Thoughts?
>>     
>
> It was my first attempt (except I didn't have a per-cpu flag, but a per-task
> flag), it's not visible but I love simplicity... ;-)
>
> A KVM VCPU is stopped by preemption, so when we enter in scheduler we have
> exited from VCPU and thus this flags is off (so we account 0 to the guest). What
> I did then is "set the flag on when we enter in the VCPU, and
> "account_system_time()" sets the flag off when it adds this timeslice to cpustat
> (and compute correctly guest, user, system time). But I didn't like this idea
> because all code executed after we entered in the VCPU is accounted to the guest
> until we have an account_system_time() and I suppose we can have real system
> time in this part. And I guess a VCPU can be less than 1 ms (unit of cputime) in
> a timeslice.
>
> So ? What's best ?
>   

The normal user/system accounting has the same issue, no?  Whereever we
happen to land (kernel or user) gets the whole tick.

So I think it is okay to have the same limitation for guest time.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.