Re: [PATCH] sched/rt: don't try to balance rt_runtime when it is futile

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, May 19, 2014 at 04:44:41AM +0200, Mike Galbraith wrote:
> On Sun, 2014-05-18 at 08:58 -0700, Paul E. McKenney wrote: 
> > On Sun, May 18, 2014 at 10:36:41AM +0200, Mike Galbraith wrote:
> > > On Sat, 2014-05-17 at 22:20 -0700, Paul E. McKenney wrote:
> > > 
> > > > If you are saying that turning on nohz_full doesn't help unless you
> > > > also ensure that there is only one runnable task per CPU, I completely
> > > > agree.  If you are saying something else, you lost me.  ;-)
> > > 
> > > Yup, that's it more or less.  It's not only single task loads that could
> > > benefit from better isolation, but if isolation improving measures are
> > > tied to nohz_full, other sensitive loads will suffer if they try to use
> > > isolation improvements.
> > 
> > So you are arguing for a separate Kconfig variable that does the isolation?
> > So that NO_HZ_FULL selects this new variable, and (for example) RCU
> > uses this new variable to decide when to pin the grace-period kthreads
> > onto the housekeeping CPU?
> 
> I'm thinking more about runtime, but yes.
> 
> The tick mode really wants to be selectable per set (in my boxen you can
> switch between nohz off/idle, but not yet nohz_full, that might get real
> interesting).  You saw in my numbers that ticked is far better for the
> threaded rt load, but what if the total load has both sensitive rt and
> compute components to worry about?  The rt component wants relief from
> the jitter that flipping the tick inflicts, but also wants as little
> disturbance as possible, so RCU offload and whatever other measures that
> are or become available are perhaps interesting to it as well.  The
> numbers showed that here and now the two modes can work together in the
> same box, I can have my rt set ticking away, and other cores doing
> tickless compute, but enabling that via common config (distros don't
> want to ship many kernel flavors) has a cost to rt performance.
> 
> Ideally, bean counting would be switchable too, giving all components
> the environment they like best.

Sounds like a question for Frederic (now CCed).  ;-)

							Thanx, Paul