On Mon, Aug 28, 2017 at 06:24:16PM +0200, Peter Zijlstra wrote: > On Mon, Aug 28, 2017 at 05:27:15PM +0200, Frederic Weisbecker wrote: > > On Mon, Aug 28, 2017 at 03:31:16PM +0200, Peter Zijlstra wrote: > > > > I'm fairly sure that was very intentional. If you want to isolate stuff > > > you don't want load-balancing. > > > > Yes I guess that was intentional. In fact having NULL domains is convenient > > as it also isolates from many things: tasks, workqueues, timers. > > Huh, what? That's entirely unrelated to the NULL domain. > > The reason people like isolcpus= is that is ensures _nothing_ runs on > those CPUs before you explicitly place something there. > > _That_ is what ensures there are no timers etc.. placed on those CPUs.
Sure that's what I meant. > > Once you run something on that CPU, it stays there. > > It is also what I dislike about isolcpus, its a boot time feature, if > you want to reconfigure your system you need a reboot. Indeed. > > > Although for example I guess (IIUC) that if you create an unbound > > timer on a NULL domain, it will be stuck on it for ever as we can't > > walk any hierarchy from the current CPU domain. > > Not sure what you're on about. Timers have their own hierarchy. Check out get_nohz_timer_target() which relies on scheduler hierarchies to look up a CPU to enqueue an unpinned timer on. > > > I'm not sure how much that can apply to unbound workqueues > > as well. > > Well, unbound workqueued will not immediately end up on those CPUs, > since they'll have an affinity exlusive of those CPUs per construction. Ah that's right. > But IIRC there's an affinity setting for workqueues where you could > force it on if you wanted to. Yep: /sys/devices/virtual/workqueue/cpumask > > > But the thing is with NULL domains: things can not migrate in and neither > > can them migrate out, which is not exactly what CPU isolation wants. > > No, its exactly what they want. You get what you put in and nothing > more. If you want something else, use cpusets. That's still a subtle behaviour that involves knowledge of some scheduler core details. I wish we hadn't exposed such a low level scheduler control as a general purpose kernel parameter. Anyway at least that confirms one worry we had: kernel parameters are kernel ABI that we can't break. > > > > Now, I completely hate the isolcpus feature and wish is a speedy death, > > > but replacing it with something sensible is difficult because cgroups > > > :-( > > > > Ah, that would break cgroup somehow? > > Well, ideally something like this would start the system with all the > 'crap' threads in !root cgroup. But that means cgroupfs needs to be > populated with at least two directories on boot. And current cgroup > cruft doesn't expect that. Ah I see. Thanks!
