Re: [PATCH 4/4] irqtime: drop local_irq_save/restore from irqtime_account_irq

Frederic Weisbecker
2016-07-08T23:58:56Z

On Fri, Jul 08, 2016 at 11:56:49AM -0400, Rik van Riel wrote:
> On Fri, 2016-07-08 at 16:34 +0200, Paolo Bonzini wrote:
> > I'm looking at an upstream tree, without your patches applied,
> > but it seems to me that irqtime_account_irq is always called with
> > interrupts disabled:
> >
> > irqtime_account_irq
> > -> account_irq_enter_time
> > -> __irq_enter
> > -> HARDIRQ_ENTER [1]
> > -> irq_enter [3]
> > -> __do_softirq [1]
> > -> account_irq_exit_time
> > -> __do_softirq [1]
> > -> __irq_exit
> > -> HARDIRQ_EXIT [1]
> > -> irq_exit [2]
> >
> > [1] = does local_irq_disable/enable
> > [2] = contains WARN_ON_ONCE(!irqs_disabled())
> > [3] = calls rcu_irq_enter(), which checks irqs_disabled()
> >
> > I don't think your first two patches change this, so perhaps it's
> > enough
> > to remove that local_irq-save/restore?  Either this, or
> > ENEEDWEEKEND...

Good catch Paolo!

>
> I think you are right!
>
> __do_softirq() calls account_irq_enter_time() with irqs
> already disabled, and also has irqs disabled when it
> calls account_irq_exit_time()
>
> This appears to be true for both ksoftirqd and softirq
> from irq context.

Indeed! And irq_enter()/irq_exit() have irqs disabled requirements.

The other users are __irq_enter() and __irq_exit() called by
lockdep selftests which takes care about it too.

I just did a boot test with a WARN_ON_ONCE(!irqs_disabled())
on account_irq_enter_time() and it triggered no issue.

>
> This could simplify my patch series a lot :)

Definetly! ;-)

Would you mind resending it?

Thanks.