Re: [PATCH 2/6] Add IRQ_TIME_ACCOUNTING, finer accounting of CPU irq time

[Peter Zijlstra <peterz@infradead.org>]

On Sun, 2010-09-19 at 13:21 +0200, Peter Zijlstra wrote:
> On Thu, 2010-09-16 at 18:56 -0700, Venkatesh Pallipadi wrote:
> > +void account_system_vtime(struct task_struct *tsk)
> > +{
> > +       unsigned long flags;
> > +       int cpu;
> > +       u64 now, delta;
> > +
> > +       if (!sched_clock_irqtime)
> > +               return;
> > +
> > +       local_irq_save(flags);
> > +
> > +       cpu = task_cpu(tsk);
> > +       now = sched_clock();
> > +       delta = now - per_cpu(irq_start_time, cpu);
> > +       per_cpu(irq_start_time, cpu) = now;
> > +       if (hardirq_count())
> > +               per_cpu(cpu_hardirq_time, cpu) += delta;
> > +       else if (softirq_count())
> > +               per_cpu(cpu_softirq_time, cpu) += delta;
> > +
> > +       local_irq_restore(flags);
> > +} 
> 
> This seems to suggest you count time double if a hardirq hits while
> we're doing softirqs, but being as this is an incomplete api its very
> hard to tell indeed.

OK, so by virtue of calling the same function on _enter and _exit its
not incomplete, just weird.

And it won't account time double, since it uses irq_start_time to
compute deltas between invocations and will attribute that delta to only
one state.

You still do have the problem with local_bh_disable() though, since you
cannot distinguish between having bh disabled and processing softirq.

So a hardirq that hits while you have bh disabled will inflate your
softirq time.

A possible solution is to have local_bh_{disable,enable} {add,sub}
2*SOFTIRQ_OFFSET and have the processing use SOFTIRQ_OFFSET, will need a
bit of a code shuffle though.