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 10:16:50 -0700
From:	Venkatesh Pallipadi <venki@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Martin Schwidefsky <schwidefsky@...ibm.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, Sep 20, 2010 at 2:27 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 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?
>

Yes. I inherited the API from VIRT_CPU_ACCOUNTING along with this
local_bh_disable bug. Agree that we need one extra bit to handle this
case. I will take a stab at fixing this along with refresh of this
patchset if no one else has beaten me to it until then.

Thanks,
Venki
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