On Tue, 2014-05-20 at 16:53 +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. > > 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).
Yeah, that would be the most flexible (not to mention invasive). That said, users of nohz_full are likely gonna be very few and far between, so maybe no big deal. I'm just looking at it from a distro perspective, what can it, can it not do, what does it cost, to see how it would best be served once baked. > I've already been eyed by vulturous frozen sharks flying in circles above me > lately > after a few overengineering visions. :) > 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. Yeah, understood, I'm just measuring and pondering potentials. -Mike -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
