On Thu, 18 Jan 2018 04:04:43 +0100 Frederic Weisbecker <frede...@kernel.org> wrote:
> On Wed, Jan 17, 2018 at 12:38:01PM -0500, Luiz Capitulino wrote: > > On Tue, 16 Jan 2018 23:51:29 +0100 > > Frederic Weisbecker <frede...@kernel.org> wrote: > > > > > On Tue, Jan 16, 2018 at 11:52:11AM -0500, Luiz Capitulino wrote: > > > > On Tue, 16 Jan 2018 16:41:00 +0100 > > > > Frederic Weisbecker <frede...@kernel.org> 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. > > > > OK, I get it now. But then series has to un-deprecate isolcpus= otherwise > > it doesn't make sense to use it. > > Good point. Also I think you convinced me toward just applying that tick > offload > on the existing nohz kernel parameter right away, that is, to both existing > "nohz_full=" > and "isolcpus=nohz". > > After all that tick offload is an implementation detail. > > Like you said if people complain about a regression, we can still fix it > with a new option. But eventually I doubt this will be needed. > > I'll respin with that. Exciting times! Btw, I do have this problem where I have a hog app on an isolated core with isolcpus=nohz_offload,domain,... and I see top -d1 going from 100% to 0% and then back from 0% to 100% every few seconds or so. I'll debug it when you post the next version.