On Tue, Dec 16, 2014 at 03:32:28PM +0100, Thomas Gleixner wrote: > @@ -4997,6 +5025,8 @@ pick_next_task_fair(struct rq *rq, struct task_struct > *prev) > struct task_struct *p; > int new_tasks; > > + if (class_fair_disabled()) > + goto idle;
We don't want to do new idle balancing here I think, just return NULL. > again: > #ifdef CONFIG_FAIR_GROUP_SCHED > if (!cfs_rq->nr_running) > > The static key is enabled once the powerclamp mess starts. So nobody > else than powerclamp users are affected by this and rightfully so. > > Not pretty, but better than a gazillion workarounds all over the place > to make "pretending I'm idle" actually work. This is basically the > same mechanism as we have with the RT throttler, where a RT hog will > be put onto hold for some time. We just put all sched other tasks on > hold while still allowing RT tasks and everything else to work. > > Thoughts? Other than hating it on sight right? ;-) So let me try and understand the problem with the emulated idle thing better (running idle from FIFO threads). Suppose we are in nohz_full: ts->inidle ts->infullnohz ts->tick_stopped 0 1 1 valid Then the powerclamp fake idle thread comes in, this increase nr_running and will result in leaving infullnohz and will re-start the tick_stopped. 0 0 0 valid Then we 'start' the idle loop, and end up in: 1 0 1 valid No problem there, right? And it looks to be the same in reverse. I suppose the tricky bit is what happens when the cpu was idle; in that case we'll end up with 1 running thread in state: 1 0 1 valid But need to avoid ending up in: 1 1 1 BUG Which should be relatively simple by never entering nohzfull when 'idle'. However with your proposed thingy, I think we'll end up in: 1 1 1 BUG Because we don't start another thread, so infullnohz will stay valid, however we'll also be 'forced' into idle (with nr_running > 0) and stop the tick. A remote wakeup might result in nr_running going from 1->2 and seeing infullnohz == 1, try and restart the tick, while we're idle! Of course, we can fix that too, by clearing nohzfull when going 'idle', after all, nohzfull will re-establish itself automagically when the tick detects but the one task afterwards. So both cases need work, neither works out of the box afaict. But I can't see one really being better than the other either -- am I missing obvious things again? -- 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/
