Re: Interrupt routine usage not shown by top in 8.0

--- On Tue, 3/17/09, Sam Leffler <sam_at_freebsd.org> wrote:

> From: Sam Leffler <sam_at_freebsd.org>
> Subject: Re: Interrupt routine usage not shown by top in 8.0
> To: barney_cordoba_at_yahoo.com
> Cc: current_at_freebsd.org
> Date: Tuesday, March 17, 2009, 4:41 PM
> 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

I'm working with the igb and ixgbe drivers. Neither of them
use the filters, but em does. Since they are all maintained
by the same person I was curious as to why the em used filters
and the igb and ixgbe, which are supposedly higher performance
cards, use MPSAFE ithreads. 

Barney