Re: [BUG] Paravirtual time accounting / IRQ time accounting

[lwcheng]

Quoting Rik van Riel <riel@xxxxxxxxxx>:

> On 03/20/2014 11:01 AM, Glauber Costa wrote:
> > On Wed, Mar 19, 2014 at 6:42 AM,  <lwcheng@xxxxxxxxx> wrote:
>
> > > ------------
> > > [src/kernel/sched/core.c]
> > > static void update_rq_clock_task(struct rq *rq, s64 delta)
> > > {
> > >     ... ...
> > > #ifdef CONFIG_IRQ_TIME_ACCOUNTING
> > >     irq_delta = irq_time_read(cpu_of(rq)) - rq->prev_irq_time;
> > >     ... ...
> > >     rq->prev_irq_time += irq_delta;
> > >     delta -= irq_delta;
> > > #endif
> > >
> > > #ifdef CONFIG_PARAVIRT_TIME_ACCOUNTING
> > >     if (static_key_false((&paravirt_steal_rq_enabled))) {
> > >         steal = paravirt_steal_clock(cpu_of(rq));
> > >         steal -= rq->prev_steal_time_rq;
> > >         ... ...
> > >         rq->prev_steal_time_rq += steal;
> > >         delta -= steal;
> > >     }
> > > #endif
> > >
> > >     rq->clock_task += delta;
> > >     ... ...
> > > }
> > > --
> > > "delta" -> the intended increment to rq->clock_task
> > > "irq_delta" -> the time spent on serving IRQ (hard + soft)
> > > "steal" -> the time stolen by the underlying hypervisor
> > > --
> > > "irq_delta" is calculated based on sched_clock_cpu(), which is vulnerable
> > > to VM scheduling delays.
> >
> > This looks like a real problem indeed. The main problem in searching
> > for a solution, is that of course not all of the irq time is steal
> > time and vice versa. In this case, we could subtract irq_time from
> > steal, and add only the steal part time that is in excess. I don't
> > think this is 100 % guaranteed, but maybe it is a good approximation.
> >
> > Rik, do you have an opinion on this ?
>
> The other way around may be better, since steal time (when it
> happens) is likely to be of "time slice" magnitude, while irq
> time will happen more frequently, and in dozens-of-microseconds
> intervals.
>
> Furthermore, we have no way to measure what the irq time is,
> except by looking at how much real time elapsed. For steal time,
> however, the hypervisor tells us exactly how much time was stolen.
>
> That means when steal time and irq time happen simultaneously,
> the amount of steal time should always be smaller than the
> calculated irq time for that period.
>
> actual irq_time = calculated irq time - reported steal time;
>
> --
> All rights reversed

I observe that sometimes irq_time only includes "part" of steal_time.

Like you said, irq_time is in dozens-of-microseconds. In VMs, as all
devices seen are virtual ones, irq_time seems to be not as desired
as it is in physical hosts.

A quick (but not radical) solution may be:
disable CONFIG_IRQ_TIME_ACCOUNTING if CONFIG_PARAVIRT is selected.
Just adopt tick-based accounting: CONFIG_TICK_CPU_ACCOUNTING

I am thinking what irq_time really *means* in VMs.

-Luwei


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