On Wednesday, August 31, 2016 06:40:09 PM Peter Zijlstra wrote: > On Wed, Aug 31, 2016 at 06:28:10PM +0200, Thomas Gleixner wrote: > > > That is the way it's been with cpufreq and many systems (including all > > > mobile devices) rely on that to not destroy power. RT + variable cpufreq > > > is not deterministic. > > > > > > Given we don't have good constraints on RT tasks I don't think we should > > > try to strengthen the semantics there. Folks should either move to DL if > > > they want determinism *and* not-sucky power, or continue disabling > > > cpufreq if they are able to do so. > > > > RT deterministic behaviour is all about meeting the deadlines. If your > > deadline is relaxed enough that you can meet it even with the lowest cpu > > frequency then it's perfectly fine to enable cpufreq. The same logic applies > > to C-States. > > > > There are a lot of RT systems out there which enable both. If cpufreq or > > c-states cause a deadline violation because the constraints of the system > > are > > tight, then people will disable it and we need a knob for both. > > > > Realtime is not as fast as possible. It's as fast as specified. > > Sure, problem is of course that RR/FIFO doesn't specify anything so the > users are left to prod knobs. > > Another problem is that we have many semi related knobs; we have the > global RT runtime limit knob, but that doesn't affect cpufreq (maybe it > should) and cpufreq has knobs to set f_min and f_max, which again are > unaware of RT anything. > > So before we go do anything, I'd like input on what is needed and how > things should tie together to make most sense.
I totally agree. We need to know where we want to get to before deciding on which way to go. Thanks, Rafael
