Re: [GIT PULL tip/genirq] Please pull from lost-spurious-irq

From: Thomas Gleixner
Date: Mon Aug 02 2010 - 10:08:20 EST


On Fri, 30 Jul 2010, Tejun Heo wrote:

> Hello,
> On 07/29/2010 10:44 AM, Thomas Gleixner wrote:
> >> I'll ask Stephen to pull the above branch into linux-next until it
> >> shows up via tip so that we don't lose any more linux-next testing
> >> time.
> >
> > I'm working on it already and I'm currently twisting my brain around
> > the problem this patches will impose to the RT stuff, where we cannot
> > access timer_list timers from atomic regions :(
> Oh, I see. A dull last ditch solution would be just disabling it on
> CONFIG_PREEMPT_RT, but yeah that will be dull. :-(

Yup, but I have some other issues with this series as well. That thing
is massively overengineered. You mangle all the various functions into
irq_poll which makes it really hard to understand what the code does
under which circumstances.

I understand most of the problems you want to solve, but I don't like
the outcome too much.

Let's look at the various parts:

1) irq polling

- Get rid of the for_each_irq() loop in the poll timer.

You can solve this by adding the interrupt to a linked list and
let the poll timer run through the list. Also why is the simple
counter based decision not good enough ? Why do we need an extra
time period for this ?

- Reenable the interrupt from time to time to check whether the irq
storm subsided.

Generally a good idea, but we really do not want to wait for
another 10k unhandled interrupts flooding the machine. Ideally we
should read out the irq pending register from the irq chip to see
whether the interrupt is still asserted.

2) irq watch

I have to admit, that I have no clue what this thing exactly
does. After staring into the code for quite a while I came to the
conclusion that it's a dynamic polling machinery, which adjusts
it's polling frequency depending on the number of good and bad
samples. The heuristics for this are completely non obvious.

Aside of that the watch mechanism adds unneccesary code to the hard
interrupt context. There is no reason why you need to update that
watch magic in the hard interrupt context. Analysing the stats can
be done in the watch timer as well. All it takes is in


and in the watch timer function

if (ret == IRQ_HANDLED)

irq_watch should use a separate global timer and a linked list. The
timer is started when an interrupt is added to the watch mechanism
and stopped when the list becomes empty.

That's a clear separation of watch and poll and simplifies the
whole business a lot.

3) irq expect

So you basically poll the interrupt until either the interrupt
happened or the device driver timeout occured.

That's really weird. What does it solve ? Not running into the
device driver timeout routine if the hardware interrupt has been

That's overkill, isn't it ? When the hardware loses an interrupt
occasionally then why isn't the device driver timeout routine
sufficient? If it does not check whether the device has an
interrupt pending despite the fact that it has not been delivered,
then this code needs to be fixed and not worked around with weird
polling in the generic irq code.

Btw, this polling is pretty bad in terms of power savings.

- The timers are not aligned, so if there are several expects armed
you get unbatched wakeups.

- If the poll interval is smaller than the average hardware
interrupt delivery time, you add more wakeups than necessary.

- If the hardware interrupt has happened, you let the poll timer
pending, which gives us an extra wakeup for each interrupt in the
worst case.

I assume your concern is to detect and deal with flaky interrupts,
but avoiding the permanent poll when the device is not used, right?

So that can be done simpler as well. One timer and a linked list

expect_irq() adds the irq to the linked list and arms the timer, if
it is not armed already. unexpect_irq() removes the irq from the
linked list and deletes the timer when the list becomes empty.

That can reuse the list_head in irq_desc which you need for irq
poll and the timer interval should be fixed without dynamic

Keep it simple!


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