Date: Tue, 17 Mar 2009 16:03:22 -0700 (PDT) From: Barney Cordoba <barney_cordoba@yahoo.com> To: Sam Leffler <sam@freebsd.org> Cc: current@freebsd.org Subject: Re: Interrupt routine usage not shown by top in 8.0 Message-ID: <914146.31317.qm@web63901.mail.re1.yahoo.com> In-Reply-To: <49C00B0E.4030402@freebsd.org>
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--- On Tue, 3/17/09, Sam Leffler <sam@freebsd.org> wrote:
> From: Sam Leffler <sam@freebsd.org>
> Subject: Re: Interrupt routine usage not shown by top in 8.0
> To: barney_cordoba@yahoo.com
> Cc: current@freebsd.org
> Date: Tuesday, March 17, 2009, 4:41 PM
> Barney Cordoba wrote:
> >
> >
> > --- On Tue, 3/17/09, Robert Watson
> <rwatson@FreeBSD.org> wrote:
> >
> >
> >> From: Robert Watson <rwatson@FreeBSD.org>
> >> Subject: Re: Interrupt routine usage not shown by
> top in 8.0
> >> To: "Paolo Pisati"
> <p.pisati@oltrelinux.com>
> >> Cc: "Barney Cordoba"
> <barney_cordoba@yahoo.com>, current@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
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