linux-kernel - Re: [PATCH/RFC 3/4]Introduce "account modifiers" mechanism

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Message-ID: <46C56774.2030009@bull.net>
Date:	Fri, 17 Aug 2007 11:16:36 +0200
From:	Laurent Vivier <Laurent.Vivier@...l.net>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	kvm-devel <kvm-devel@...ts.sourceforge.net>,
	Ingo Molnar <mingo@...e.hu>,
	virtualization <virtualization@...ts.linux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH/RFC 3/4]Introduce "account modifiers" mechanism

Rusty Russell wrote:
> On Fri, 2007-08-17 at 09:35 +0200, Laurent Vivier wrote:
>> Rusty Russell wrote:
>>> Hi Laurent,
>> Hi Rusty,
>> how are your puppies ?
> 
> They're getting a little fat, actually.  Too many features ...
> 
>> - 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 ?
Laurent
-- 
------------- Laurent.Vivier@...l.net  --------------
          "Software is hard" - Donald Knuth


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