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

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Date:	Mon, 20 Sep 2010 09:27:34 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Venkatesh Pallipadi <venki@...gle.com>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH 2/6] Add IRQ_TIME_ACCOUNTING, finer accounting of CPU
 irq time

On Sun, 19 Sep 2010 14:01:06 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> 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.

That is the same with CONFIG_VIRT_CPU_ACCOUNTING=y. irq_enter/irq_exit
call account_system_vtime, the function then uses the preempt/softirq/
hardirq counter to find out which context is currently active.

> 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.

irq_start_time is a bit misleading, it is a time stamp of the last update.
The confusing part (which deserves a comment) is the fact that the delta
is not added to anything if hardirq_count and softirq_count are zero.

> 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.

Hmm, that bug is valid for CONFIG_VIRT_CPU_ACCOUNTING=y as well.

-- 
blue skies,
   Martin.

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

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