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 11:27:30 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
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 Mon, 2010-09-20 at 09:27 +0200, Martin Schwidefsky wrote:

> > 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.

Yeah, I realized that eventually, I've so far been able to mostly ignore
all that VIRT_CPU_ACCOUNTING muck.

> > 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.

Yeah, the name didn't help either, but I really expected to see two
hooks: start/exit, I did eventually figure it all out, but its a bit
daft.

If you would have had 4 hooks, the below problem would have been fixable
within the implementation.

> > 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.

And nobody ever noticed?
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