On Tue, Jan 16, 2018 at 11:52:11AM -0500, Luiz Capitulino wrote: > On Tue, 16 Jan 2018 16:41:00 +0100 > Frederic Weisbecker <[email protected]> wrote: > > So isolcpus= is now the place where we control the isolation features > > and nohz is one of them. > > That's the part I'm not very sure about. We've been advising users to > move away from isolcpus= when possible, but this very wanted nohz_offload > feature will force everyone back to using isolcpus= again.
Note "isolcpus=nohz" only implies nohz. You need to add "domain" to get the behaviour that you've been advising users against. We are simply reusing a kernel parameter that was abandoned to now control the isolation features that were disorganized and opaque behind nohz. > > I have the impression this series is trying to solve two problems: > > 1. How (and where) we control the various isolation features in the > kernel No, that has already been done in the previous merge window. We have a dedicated isolation subsystem now (kernel/sched/isolation.c) and an interface to control all these isolation features that were abusively implied by nohz. The initial plan was to introduce "cpu_isolation=" but it looked too much like "isolcpus=". Then in fact, why not using "isolcpus=" and give it a second life. And there we are. In the end the goal is to propagate what is passed to "isolcpus=" to cpusets. > > 2. Where we add the control for the tick offload feature > > I think item 1 is too complex to solve right now. IMHO, this series > should focus on item 2. And regarding item 2, I think we have two > choices to make: > > 1. Make tick offload a first class citizen by making it default to > nohz_full=. If there are regressions, we handle them That's a possible way to go. > > 2. Add a new option to nohz_full=, like nohz_full=tick_offload > > As an avid user of nohz_full I'm dying to see option 1 happening, > but I'm not totally sure what the consequences can be. "nohz_full=" parameter has been badly designed as it implies much more than just full dynticks. So I'm not really looking forward to expanding it. > Another idea is to add CONFIG_NOHZ_TICK_OFFLOAD as an experimental > feature. I fear it's way too distro-unfriendly. They will want to have it as a capability without necessarily running it. Just like they do with CONFIG_NO_HZ_FULL. > > > The complain about isolcpus is the immutable result. I'm thinking about > > making it modifiable to cpuset but I only see two possible solutions: > > > > - Make the root cpuset modifiable > > - Create a directory called "isolcpus" visible on the first cpuset mount > > and move all processes there. > > So, if we move the control of the tick offload to nohz_full= itself, > we can completely ditch any isolcpus= change in this series. > > I think this should give you a great relief :) Not at all :) What would be a great relief to me is that we can finally propagate isolcpus= to cpusets so that we can continue to expand it without a second thought.
