Interrupt routine usage not shown by top in 8.0

[Sam Leffler]

Barney Cordoba wrote:
>
>
> --- On Tue, 3/17/09, Robert Watson <rwatson at FreeBSD.org> wrote:
>
>   
>> From: Robert Watson <rwatson at FreeBSD.org>
>> Subject: Re: Interrupt routine usage not shown by top in 8.0
>> To: "Paolo Pisati" <p.pisati at oltrelinux.com>
>> Cc: "Barney Cordoba" <barney_cordoba at yahoo.com>, current at freebsd.org
>> Date: Tuesday, March 17, 2009, 11:24 AM
>> On Tue, 17 Mar 2009, Paolo Pisati wrote:
>>
>>     
>>> perhaps i misunderstood your question, but i'll
>>>       
>> try to explain a bit:
>>     
>>> before 7.0, bus_setup_intr() took just one function
>>>       
>> thus you could have an INTR_FAST or an INTR_MPSAFE handler,
>> and you choose the kind of handler via a flag (INTR_FAST in
>> this case).
>>     
>>> after 7.0, bus_setup_intr() took 2 functions, thus you
>>>       
>> could have: a fast handler (aka filter), or an ithread
>> handler (aka mpsafe), or a fast + ithread handler (available
>> only with INTR_FILTER turned on).
>>     
>>> in bus_setup_intr() the first function pointer is for
>>>       
>> the filter side of the handler, while the second pointer is
>> for the ithread part, and if you declare both you can filter
>> events (interrupts) and call the rest of the device driver
>> (the ithread part) after the filter has recognized and
>> acknowledged&masked the interrupt.
>>
>> This clarifies my misunderstanding, thanks!
>>
>>     
>
> I'd still be interested in knowing the specific advantage/consequences
> of a fast filter vs an MPSAFE ithread?
>
> In what circumstance would using a filter and then launching a task be 
> advantageous over just using an ithread?
>   

It mostly depends on the hardware (unless the fast handler does actual 
work).  If ack'ing the interrupt improves latency (e.g. by allowing the 
device to do other things) then it's better to do that in the filter 
method even if the actual work is deferred to the ithread.  It's also 
important when interrupts are not edge-triggered; you want to shut them 
up asap.

So, what device are you doing a driver for?

    Sam