Re: [PATCH v2] kvm: Fix accounting of interrupts during guest execution on s390

[Christian Borntraeger <borntraeger-tA70FqPdS9bQT0dZR+AlfA@public.gmane.org>]

Am Donnerstag, 15. November 2007 schrieb Laurent Vivier:
> If I remember correctly time accounting on s390 is more accurate than on
> x86 ?

Yes. its done during context switches and resolution is microsecond.

> Because on x86, as we make the kvm_guest_exit() after local_irq_enable()
> we can also have IRQ with PF_VCPU set... and we discussed a lot on
> probability to know if it was good or not. And on x86 it seems good
> because it is already working like that with system and user time (we
> account time to the space where tick appears).
> see http://lkml.org/lkml/2007/10/15/228

I am not sure I fully understand your point, can you try to explain?

My patch deals with timer ticks (see hardirq_offset). So if a only a tick
comes in after local_irq_enable the time is accounted to guest time as 
before.
I made a test on x86_64 with burnP6 inside a kvm machine. top showed 100%
guest time on an otherwise idle host. So the guest  accounting itself did
still work. I tried some disk and network stress on the host but I did not
manage to bring hardirq+softirq time above 5%, guest time stayed above
90%. At least my simple testcase did not show a bias towards irq time.

> Le jeudi 15 novembre 2007 à 15:10 +0100, Christian Borntraeger a écrit :
> > Avi pointed out, that my first patch was broken, here is the 2nd try.
> > I tested the patch on s390 with CONFIG_VIRT_CPU_ACCOUNTING and on x86_64.
> > Seems to work.
> > 
> > Currently the scheduler checks for PF_VCPU to decide if this
> > timeslice has to be accounted as guest time. On s390 host 
> > interrupts are not disabled during guest execution. This causes
> > theses interrupts to be accounted as guest time if
> > CONFIG_VIRT_CPU_ACCOUNTING is set.
> > Solution is to check if an interrupt triggered account_system_time.
> > As the tick is timer interrupt based, we have to subtract
> > hardirq_offset.
> > 
> > Avi, Ingo, Laurent, feedback is welcome.
> > 
> > CC: Ingo Molnar <mingo-X9Un+BFzKDI@public.gmane.org>
> > CC: Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>
> > CC: Laurent Vivier <Laurent.Vivier-6ktuUTfB/bM@public.gmane.org>
> > Signed-off-by: Christian Borntraeger <borntraeger-tA70FqPdS9bQT0dZR+AlfA@public.gmane.org>
> > ---
> >  kernel/sched.c |    6 ++----
> >  1 file changed, 2 insertions(+), 4 deletions(-)
> > 
> > Index: kvm/kernel/sched.c
> > ===================================================================
> > --- kvm.orig/kernel/sched.c
> > +++ kvm/kernel/sched.c
> > @@ -3395,10 +3395,8 @@ void account_system_time(struct task_str
> >  	struct rq *rq = this_rq();
> >  	cputime64_t tmp;
> >  
> > -	if (p->flags & PF_VCPU) {
> > -		account_guest_time(p, cputime);
> > -		return;
> > -	}
> > +	if ((p->flags & PF_VCPU) && (irq_count() - hardirq_offset == 0))
> > +		return account_guest_time(p, cputime);
> >  
> >  	p->stime = cputime_add(p->stime, cputime);

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