RE: [patch] voluntary-preempt-2.6.8-rc2-L2, preemptable hardirqs

[Saksena, Manas]

Lee Revell wrote:
> On Tue, 2004-07-27 at 12:27, Ingo Molnar wrote:
>> i've uploaded -L2:
>> 
>> 
>> 
> http://redhat.com/~mingo/voluntary-preempt/voluntary-preempt-2.6.8-rc2
>> -L2
>> 
>> the hardirq-redirection feature is activated via
>> voluntary-preemption=3 (default). All irqs except the timer irq are
>> redirected. (the timer irq needs to run from irq context - but it has
>> constant latency and constant frequency so it's not an issue. Soft
>> timers are executed from the timer softirq which is redirected and
>> hence preemptable.) 
>> 
>> this means that with this patch applied the 2.6 UP kernel is quite
>> close to being hard-RT capable (using controlled drivers and
>> workloads). All unbound-latency asynchronous workloads have been
>> unloaded into synchronous, schedulable process contexts - so nothing
>> can delay a high-prio RT task. (assuming we caught all the latencies.
>> Any remaining latency can be fixed with the existing methods.)
>> 
> The obvious next feature to add would be to make certain IRQs
> non-schedulable, like you would for an RT system.  For an
> audio system this would be just the soundcard interrupt (and timer as
> stated above). Then, while it still might not be hard-RT, it would
> blow away anything achievable on the other OS'es people do audio work
> with. 

Its really useful to have a thread per IRQ and to have an option to
run a particular IRQ handler in hardirq context. Then a system designer
or adminstrator can tune the priorities of different IRQ handler threads
depending on the latency requirements of a particular system. Scott's 
patch posted earlier (see link below) implements this -- this has proven

handy in many scenarios including, but not restricted to, audio
real-time 
performance. 

http://marc.theaimsgroup.com/?l=linux-kernel&m=109096903816850&w=2

Manas
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