Re: [Ksummit-discuss] [TECH TOPIC] Mainlining PREEMPT_RT

["Luis R. Rodriguez" <mcgrof@suse.com>]

On Tue, Oct 13, 2015 at 06:39:34PM -0400, Steven Rostedt wrote:
> On Wed, 14 Oct 2015 00:19:41 +0200
> "Luis R. Rodriguez" <mcgrof@suse.com> wrote:
> 
> > On Wed, Oct 14, 2015 at 12:40:10AM +0200, Rafael J. Wysocki wrote:
> > > On Tuesday, October 13, 2015 12:42:30 PM Steven Rostedt wrote:
> > > > 
> > > > Now that it was announced in Dublin last week that mainlining
> > > > PREEMPT_RT has funding, I would like to propose a tech topic to discuss
> > > > what is needed to be done in the near future.
> > > > 
> > > > Thomas Gleixner and his minions will be doing the brunt of the work,
> > > > but there's issues that need to be addressed with the rest of the
> > > > kernel. The various topics would include:
> > > > 
> > > >  CPU hotplug
> > > >  preempt_disable and local_irq_disable annotations
> > > >  new timer_wheel implementation
> > > >  simple waitqueues
> > > >  trylock boosting
> > > > 
> > > > Several of the Real Time developers will be present at kernel summit. I
> > > > think this would be a great opportunity to have them be able to sit
> > > > down together and discuss the remaining issues.
> > > 
> > > Well, that's not limited to the RT developers even.
> > > 
> > > Good topic.
> > 
> > How much of this is the above topics Vs lack of good semantics in general, or
> > code which needs love / good design? I ask as we have a few 'semantic'
> > discussions, and I have a feeling there might be overlap.
> 
> Well, CPU hotplug needs a rewrite. Preempt disabling and irq disabling
> needs some commentary on what they are actually protecting. The timer
> wheel is showing its age. Simple waitqueues can help make the majority
> of the cases less complex. But the trylock boosting is -rt only.
> 
> As always, when -rt goes into mainline, we try hard to make sure those
> changes improve mainline. Thus, we change designs and clean up code as
> -rt goes in. We've been doing that since 2004, and nobody took
> notice ;-)  That is, people saw improvements and never knew those
> improvements were solely to get -rt working better.

That's what I was alluding to -- what lessons have been learned here
that we have needed over time to make that happen, which to others
not in the know, perhaps have been transparent / nothing new other
than yet-another-kernel-feature / kernel-bell-whistle.

  Luis