linux-kernel - Re: [PATCH 1/7] si time accounting accounts bh

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Date:	Sun, 03 Oct 2010 02:34:16 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Venkatesh Pallipadi <venki@...gle.com>
Cc:	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	linux-kernel@...r.kernel.org, Paul Turner <pjt@...gle.com>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH 1/7] si time accounting accounts bh_disable'd time to
 si -v3

On Sat, 2010-10-02 at 08:42 -0700, Venkatesh Pallipadi wrote:
> 
> > The make in_serving_softirq() be something like:
> >  (preempt_count() & SOFTIRQ_OFFSET) || (current->flags & PF_SOFTIRQ)
> >
> 
> Yes. I would also need in_softirq_at_hardirq_tail() for accounting calls :)

Well, you could open-code it there or something.

> >> Also, ksoftirqd adds to softirq counts, does trace softirq, etc. So,
> >> it kind of made sense to add the time also to softirq stats as well.
> >> If we dont account time to softirq stats, then if some user is looking
> >> at say time per softirq using the softirq count will be misled. No?
> >
> > Simply add back the task accounting when you report it?
> >
> >> In the other thread you mentioned doing that will cause problems. Were
> >> you thinking of scheduler issues or are there other problems charging
> >> softirq time this way?
> >
> > Of course there are.. you're double accounting the time of ksoftirqd,
> > and worse, you're adding that back into the equation as part of the !
> > sched_fair time.
> >
> 
> No. There should not be any double accounting with this current
> change. We account softirq processing both at hardirq tail and
> ksoftirqd as CPU softirq time. It will be taken out of ksoftirqd sched
> exec time as with any other thread. And it will be taken out of fair
> time available on the CPU as well. Which to me seems to be the right
> thing to do, as the this is more coupled with the CPU and ksoftirqd is
> just giving context for softirqd to run.

But ksoftirqd is a SCHED_OTHER task, so by taking it out of its runtime
the scheduler will get all confused.

> Changing only hardirq tail to have SOFTIRQ_OFFSET and changing
> ksoftirqd to SOFTIRQ_OFFSET*2 would result in these additional
> ksoftirqd softirqs staying as the part of sched_fair time.
> 
> Or I am totally missing something here? 

Please keep ksoftirq scheduling normal, if people want it to be another
scheduling class, let them change that, but since its a task it should
be a normal task and get scheduled like everybody else, not have some
magic properties.


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