On Wed, Jun 19, 2013 at 04:39:39PM +0100, Arjan van de Ven wrote: > On 6/18/2013 10:47 AM, David Lang wrote: > > > > > It's bad enough trying to guess the needs of the processes, but if you also > > are reduced to guessing the capabilities of the cores, how can anything be > > made to work? > > btw one way to look at this is to assume that (with some minimal hinting) > the CPU driver will do the right thing and get you just about the best > performance you can get > (that is appropriate for the task at hand)... > ... and don't do anything in the scheduler proactively.
If I understand correctly, you mean if your hardware/firmware is fully in control of the p-state selection and changes it fast enough to match the current load, the scheduler doesn't have to care? By fast enough I mean, faster than the scheduler would notice if a cpu was temporarily overloaded at a low p-state. In that case, you wouldn't need cpufreq/p-state hints, and the scheduler would only move tasks between cpus when cpus are fully loaded at their max p-state. > > Now for big.little and other temporary or permanent asymmetries, we may want > to > have a "max performance level" type indicator, and that's fair enough > (and this can be dynamic, since it for thermal reasons this can change over > time, > but on a somewhat slower timescale) > > > the hints I have in mind are not all that complex; we have the biggest issues > today > around task migration (the task migrates to a cold cpu... so a simple > notifier chain > on the new cpu as it is accepting a task and we can bump it up), real time > tasks > (again, simple notifier chain to get you to a predictably high performance > level) > and we're a long way better than we are today in terms of actual problems. > > For all the talk of ondemand (as ARM still uses that today)... that guy puts > you in > either the lowest or highest frequency over 95% of the time. Other > non-cpufreq solutions > like on Intel are bit more advanced (and will grow more so over time), but > even there, > in the grand scheme of things, the scheduler shouldn't have to care anymore > with those > two notifiers in place. You would need more than a few hints to implement more advanced capacity management like proposed for the power scheduler. I believe that Intel would benefit as well from guiding the scheduler to idle the right cpu to enable deeper idle states and/or enable turbo-boost for other cpus. Morten -- 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/
