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

[Mike Galbraith]

On Tue, 2014-05-20 at 08:53 -0700, Paul E. McKenney wrote: 
> On Tue, May 20, 2014 at 04:53:52PM +0200, Frederic Weisbecker wrote:
> > On Sun, May 18, 2014 at 10:34:01PM -0700, Paul E. McKenney wrote:
> > > 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).  ;-)
> > 
> > I'm not sure that I really understand what you want here.
> > 
> > The current state of the art is that when you enable CONFIG_NO_HZ_FULL=y, full dynticks
> > is actually off by default. This is only overriden by "nohz_full=" boot parameter.
> 
> If I understand correctly, if there is no nohz_full= boot parameter,
> then the context-tracking code takes the early exit via the
> context_tracking_is_enabled() check in context_tracking_user_enter().
> I would not expect this to cause much in the way of syscall performance
> degradation.  However, it looks like having even one CPU in nohz_full
> mode causes all CPUs to enable context tracking.
> 
> My guess is that Mike wants to have (say) half of his CPUs running
> nohz_full, and the other half having fast system calls.  So my guess
> also is that he would like some way of having the non-nohz_full CPUs
> to opt out of the context-tracking overhead, including the memory
> barriers and atomic ops in rcu_user_enter() and rcu_user_exit().  ;-)

Bingo.

> > Now if what you need is to enable or disable it at runtime instead of boottime,
> > I must warn you that this is going to complicate the nohz code a lot (and also perhaps sched
> > and RCU).
> 
> What Frederic said!  Making RCU deal with this is possible, but a bit on
> the complicated side.  Given that I haven't heard too many people complaining
> that RCU is too simple, I would like to opt out of runtime changes to the
> nohz_full mask.
> 
> > I've already been eyed by vulturous frozen sharks flying in circles above me lately
> > after a few overengineering visions.
> 
> Nothing like the icy glare of a frozen shark, is there?  ;-)
> 
> > And given that the full nohz code is still in a baby shape, it's probably not the right
> > time to expand it that way. I haven't even yet heard about users who crossed the testing
> > stage of full nohz.
> > 
> > We'll probably extend it that way in the future. But likely not in a near future.
> 
> My guess is that Mike would be OK with making nohz_full choice of CPUs
> still at boot time, but that he would like the CPUs that are not to be
> in nohz_full state be able to opt out of the context-tracking overhead.
> 
> Mike, please let us all know if I am misunderstanding what you are
> looking for.

Yup, exactly.  As it sits, you couldn't possible ship nohz_full out to
the real world in any other form than a specialty kernel.  There's no
doubt in my mind that there are users out there who would love to have
high performance rt and compute in the same box though.  I can imagine
them lurking here and slobbering profusely ;-)

-Mike

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