Threaded interrupts and preemption

[Daniel J Blueman]

On Tue, Apr 13, 2010 at 7:11 AM, Chase Douglas
<chase.douglas at canonical.com> wrote:
> On Mon, Apr 12, 2010 at 10:56 PM, Alessio Igor Bogani
> <abogani at ubuntu.com> wrote:
>> Hi,
>>
>> 2010/4/13 Chase Douglas <chase.douglas at canonical.com>:
>> [...]
>>> specifically set it up otherwise. I think it would be fun to have an
>>> RT completely preemptible kernel just so we can try it out and have a
>>> comparison. We would gain an insight as to whether a responsiveness
>>> issue is scheduler algorithm based or latency based.
>>
>> sudo apt-get install linux-rt
>
> As far as I know, our -rt isn't what everyone else means when they say
> RT. Our rt flavor just has full preemption and a higher tick rate.
> That's nothing compared to what is in the RT patch set maintained by
> tglx at http://www.kernel.org/pub/linux/kernel/projects/rt/. The
> biggest changes that I'm aware of is the default threaded irq handlers
> and complete preemption in his patch set. Ubuntu kernels since at
> least Karmic have had threaded irq handlers, but the driver has to be
> reworked for it.
>
> I'll probably look into branching the Lucid kernel and applying the
> .33 RT patch set, backporting where necessary. Again, I think this is
> more to help us gauge how a desktop system behaves on an RT
> low-latency kernel than to be a test for wider use at this point.
> According to the commit message for the threaded irq handlers in
> mainline the RT approach of defaulting to threaded handlers is a blunt
> tool and some softirq/tasklet interactions may cause issues. However,
> if we find that the RT patch set works wonders we can potentially use
> some latency benchmarking tools to determine what is causing extra
> latency. Maybe we can find the top latency causing irqs and port them
> to threaded handlers. It doesn't seem too difficult to do since many
> drivers already just work with the RT patch set.

I think offering a realtime-enabled kernel does bring some wins to
Ubuntu, for example Novell's SuSE Linux Enterprise Realtime is the
only mainstream offering right now (and I believe is only the kernel),
though it does come with expense to normal throughput.

On another angle, the faster processors and memory are, the relatively
longer is spent synchronously waiting for disk, network, timers,
MMIO/PIO register reads/barriers etc, and there is some benefit in
using the '-rt' kernel with due to the preemption outside spinlocks
and atomic regions.
-- 
Daniel J Blueman