Re: [ANNOUNCE] Linux 2.6 Real Time Kernel

[Ingo Molnar <mingo@elte.hu>]

* Dipankar Sarma <dipankar@in.ibm.com> wrote:

> On Tue, Oct 12, 2004 at 07:50:29AM +0200, Ingo Molnar wrote:
> > 
> > regarding RCU serialization - i think that is the way to go - i dont
> > think there is any sensible way to extend RCU to a fully preempted
> > model, RCU is all about per-CPU-ness and per-CPU-ness is quite limited 
> > in a fully preemptible model.
> 
> It seems that way to me too. Long ago I implemented preemptible RCU,
> but did not follow it through because I believed it was not a good
> idea. The original patch is here :
> 
> http://www.uwsg.iu.edu/hypermail/linux/kernel/0205.1/0026.html

interesting!

> This allows read-side critical sections of RCU to be preempted. It
> will take a bit of work to re-use it in RCU as of now, but I don't
> think it makes sense to do so. [...]

note that meanwhile i have implemented another variant:

  http://marc.theaimsgroup.com/?l=linux-kernel&m=109771365907797&w=2

i dont think this will be the final interface (the _rt postfix is
stupid, it should probably be _spin?), but i think this is roughly the
structure of how to attack it - a minimal extension to the RCU APIs to
allow for serialization. What do you think about this particular
approach?

> [...] My primary concern is DoS/OOM situation due to preempted tasks
> holding up RCU.

in the serialization solution in -U0 it would be possible to immediately
free the RCU entries and hence have no DoS/OOM situation - although the
-U0 patch does not do this yet.

	Ingo