Re: Question on RCU_BOOST option

[Thomas Gleixner <tglx@linutronix.de>]

On Wed, 27 Mar 2013, Koehrer Mathias (ETAS/ESS2) wrote:
> > 
> > Perhaps you might want to have a look at this thread, about RCU stalls
> > and a possible common root cause for some of them.
> > 
> > http://marc.info/?l=linux-rt-users&m=136258510132233&w=2
> 
> Thanks for the information. However the application is intended to
> run with 100% CPU (real time) load on one core.  That's why the CPU
> core is isolated for. They are reserved for this application.

Sure, but it's not possible today to run 100% CPU in user space. RCU
is one of the issues, which has a solution in 3.8. See
https://lwn.net/Articles/522262/

There is work in progress to solve the other issues as well, but
that's not going to happen before 3.10.

> The one option is to raise the prioriy of ksoftirq (which contains
> the RCU stuff (if I know right...)) or I was playing with the
> RCU_BOOST feature which I expected to have the same effect.

RCU_BOOST does not solve that. It's about starvation of readside
critical sections.  See https://lwn.net/Articles/220677/

> However, the RCU_BOOST feature is not clear to me as I still saw the
> kernel messages about RCU stalls that I did not want to see
> anymore...

Sure, RCU_BOOST is tackling a different problem. The callback free
CPUs implementation is what you are looking for.

Thanks,

	tglx