Re: rt20 patch question

[Steven Rostedt <rostedt@goodmis.org>]

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On Tue, 9 May 2006, Mark Hounschell wrote:

> Daniel Walker wrote:
> > On Tue, 2006-05-09 at 08:23 -0400, Mark Hounschell wrote:
> >> Can I assume configuring 'Complete preemption' is the same as
> >> configuring ('Voluntary preemption' + 'Hardirq' + 'Softirq' + default
> >> proc settings)?
> >
> > Not Voluntary preemption, and I'm not sure what default proc settings is
> > referring too .
>
> The proc settings or boot options to enable or disable hardirq or
> softirq threading that you have avaialable in Voluntary preemption.
>
> > Complete preemption is like CONFIG_PREEMPT and softirq
> > and hardirq threading .. The preemption isn't voluntary, it's forced .
> >
>
> Complete preemption you have no choice of threading hard or soft irqs.
> They are threaded.
>
> So If I config Voluntary preemption + Hardirq and Softirq threading and
> do not disable hardirq or softirq via proc or boot cmdline, is that the
> same as configuring Complete preemption?
>

No not at all.

First Voluntary preemption means that when you are executing in the
kernel, and a higher priority process needs to run, the kernel will _not_
be preempted!  Voluntary preemption means that there are places in the
kernel that are marked as preemption points.  So if you are in the kernel
and you hit a preemption point, a check is made then to see if the
scheduler should be called. So, really this is not a true preemptive
kernel.

Next you have "Preemptible Kernel (Low Latency Desktop)".  This _is_ a
preemptive kernel.  Which means that, unless preemption is disabled, the
default is to preempt a process whether or not it's in the kernel if
either it finished it's run time, or a higher priority process wants to
run.  There is protective places in the kernel that disallow preemption
(basically between spinlocks and preempt_enable/disable).

But even with Preemptible Kernel + Hardirq and Softirq threading, you
still don't have the same as complete preemption.  This is because the
full preemption turns the spinlocks into mutexes that are not only
preemptible, but schedule on contention.  To do this, Hard and Soft irqs
must be threaded.  This is because they use spinlocks, and to schedule
in an interrupt, it must be acting as a thread.  So you can't have full
complete preemption without threading the Hard and Soft irqs, and that's
why there is no option to not have them threaded.

Without full preemption, you also lose out on having the PI for the
spinlock mutexes.

-- Steve

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