linux-kernel - Re: [PATCH 0/4] Virtual Machine Time Accounting

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Date:	Tue, 4 Aug 2009 19:29:24 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Laurent Vivier <Laurent.Vivier@...l.net>,
	Ingo Molnar <mingo@...e.hu>,
	kvm-devel <kvm-devel@...ts.sourceforge.net>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	virtualization <virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH 0/4] Virtual Machine Time Accounting

On Tue, 04 Aug 2009 18:26:41 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, 2009-08-04 at 17:07 +0200, Martin Schwidefsky wrote:
> > On Tue, 04 Aug 2009 16:16:38 +0200
> > Peter Zijlstra <peterz@...radead.org> wrote:
> > 
> > > These patches never seem to have made it onto LKML?!
> > > 
> > > On Mon, 2007-08-20 at 15:13 +0200, Laurent Vivier wrote:
> > > > The aim of these four patches is to introduce Virtual Machine time accounting.
> > > > 
> > > > _Ingo_, as these patches modify files of the scheduler, could you have a look to
> > > > them, please ?
> > > > 
> > > > [PATCH 1/4] as recent CPUs introduce a third running state, after "user" and
> > > > "system", we need a new field, "guest", in cpustat to store the time used by
> > > > the CPU to run virtual CPU. Modify /proc/stat to display this new field.
> > > > 
> > > > [PATCH 2/4] like for cpustat, introduce the "gtime" (guest time of the task) and
> > > > "cgtime" (guest time of the task children) fields for the
> > > > tasks. Modify signal_struct and task_struct. Modify /proc/<pid>/stat to display
> > > > these new fields.
> > > > 
> > > > [PATCH 3/4] modify account_system_time() to add cputime to cpustat->guest if we
> > > > are running a VCPU. We add this cputime to  cpustat->user instead of
> > > > cpustat->system because this part of KVM code is in fact user code although it
> > > > is executed in the kernel. We duplicate VCPU time between guest and user to
> > > > allow an unmodified "top(1)" to display correct value. A modified "top(1)" is
> > > > able to display good cpu user time and cpu guest time by subtracting cpu guest
> > > > time from cpu user time. Update "gtime" and "cgtime" in signal_struct and
> > > > task_struct accordingly.
> > > > 
> > > > [PATCH 4/4] Modify KVM to update guest time accounting.
> > > 
> > > Isn't this exactly what CONFIG_VIRT_CPU_ACCOUNTING is about?
> > 
> > Not really, CONFIG_VIRT_CPU_ACCOUNT is a mechanism to sort out the
> > steal time in the >guest< and to increase the precision of the cpu
> > accounting values in general. The patches from Laurent improve the
> > code in the >host< that sorts out guest time vs. system time.
> 
> Ah, that wasn't clear, it keeps mentioning guest all over the place ;-)
> 
> So its going to split user time into user and guest. Does that really
> make sense? For the host kernel it really is just another user process,
> no?

The code (at least in parts) is already upstream. Look at the
account_guest_time function:

static void account_guest_time(struct task_struct *p, cputime_t cputime,
                               cputime_t cputime_scaled)
{
        cputime64_t tmp;
        struct cpu_usage_stat *cpustat = &kstat_this_cpu.cpustat;

        tmp = cputime_to_cputime64(cputime);

        /* Add guest time to process. */
        p->utime = cputime_add(p->utime, cputime);
        p->utimescaled = cputime_add(p->utimescaled, cputime_scaled);
        account_group_user_time(p, cputime);
        p->gtime = cputime_add(p->gtime, cputime);

        /* Add guest time to cpustat. */
        cpustat->user = cputime64_add(cpustat->user, tmp);
        cpustat->guest = cputime64_add(cpustat->guest, tmp);
}

The cpu time for a guest is added to p->utime AND p->gtime. That is
done not to break existing tools that know nothing about guest time.
A guest time aware tool can subtract the p->gtime from p->utime to
get the time spent by the process outside of the guest context.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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