Dan Mick wrote:
> Garrett D'Amore wrote:
>> I'm getting ready to write the kstat intr support for a revised audio
>> driver, and I noticed that it only reports hard, not spurious
>> interrupts. That got me thinking.
>>
>> On an x86 system at least, the interrupt chain logic keeps retrying
>> the interrupt until all devices report DDI_INTR_UNCLAIMED. So, a
>> typical device on an unshared bus will have at least 1 spurious
>> interrupt for each hard interrupt. (Ignoring the case where
>> successive calls into the drivers ISR return DDI_INTR_CLAIMED, of
>> course.)
>
> Well...there's "I got an interrupt indication, but I can't tell what
> device it's for" and there's "I'm looping to look for any other
> claimers because of ancient sharing rules", and I don't think they
> really both fall into the class of what I'd call "spurious", but...I'm
> not sure what you mean by spurious, or the need/desire to count them.
>
>> On a system with shared interrupts (more and more common,
>> *especially* on x86 systems) the spurious interrupt count is likely
>> to be much higher for some devices, because a busy device will cause
>> spurious interrupts (or the appearance thereof) in the less busy
>> device(s) on the same interrupt.
>>
>> The upshot of all this is, it seems to me that drivers cannot
>> reliably count spurious interrupts.
>>
>> The other observation is that very few drivers bother to use
>> kstat_intr. (Some drivers, such as NIC drivers, report interrupts
>> differently, such as in a named kstat.)
>>
>> There's the intrstat program which uses dtrace, but from what I can
>> tell, it seems to record entries into the interrupt handler, which
>> doesn't correctly count hard interrupts (see above for why not!)
>
> By "hard interrupts" you mean "an actual hardware indication of
> interrupt"?
> Note that, for level-triggered interrupts, you can't ever really count
> those accurately anyway, but...yeah, it counts ISR activations. While
> I'm not saying this is right or wrong, what do you see as a problem
> with this?
>
>> So what I'm thinking is that the interrupt framework itself could
>> count interrupts and keep track of this in a kstat. This is useful
>> for debug, but it would also be able to provide a more accurate
>> picture than intrstat does. (Furthermore, as much as I like dtrace,
>> it might make it possible to build a portable intrstat, that was able
>> to report not just PCI interrupts but other kinds of interrupts --
>> hard, soft, pci, pciex, isa, usb, firewire, sbus, and maybe other
>> kinds in the future (for example SDcard can support IO devices that
>> interrupt, although the framework and nexus drivers I just putback
>> into 97 don't have support for SDIO yet.)
>>
>> The framework could also count spurious interrupts -- a spurious
>> interrupt being one where the top-level CPU or nexus specific
>> interrupt handler dispatched, but didn't find *any* handler that
>> could service the interrupt. (This could be counted as spurious
>> interrupt for *each* device on the bus, or it could be counted in a
>> separate top-level spurious interrupt counter for the bus.)
>>
>> Thoughts?
>
> I've mused from time to time about trying to clean that code up just
> from the "wasteful to call everyone again and again" aspect, but it
> would change semantics; probably in a way we can handle, but I'd have
> to think harder about it. I've never thought about deficiencies in
> the stats, though, and I'm not sure one way is better or worse than
> the other.
>
> What would you gain by knowing about 'spurious', where 'spurious'
> really relates to two different possible conditions? It's a
> negatively-charged term, but one of the two types is just "the way
> Solaris does it". What would be better about counting only 'hard
> interrupts'?
Its useful to know if a device is interrupting and then not handling the
interrupts. It can also give some indication of interrupt latencies,
and "wasted" interrupt related context switches.
It also gives a good indication of how much impact your interrupt
sharing is causing you.
Hard interrupt statistics are *incredibly* useful. For example, I need
to know how many times a second a certain audio related interrupt is
occurring, so that I know that I have configured it properly.
-- Garrett
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