Messages in this thread | | | Date | Mon, 3 Jul 2023 16:32:57 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v4] Sched/fair: Block nohz tick_stop when cfs bandwidth in use |
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On Mon, Jul 03, 2023 at 10:10:56AM -0400, Phil Auld wrote: > On Mon, Jul 03, 2023 at 02:10:09PM +0200 Peter Zijlstra wrote: > > On Fri, Jun 30, 2023 at 12:29:10PM -0400, Phil Auld wrote: > > > > > I think you are agreeing that I need the pick next code but need to remove > > > the hierarchy walks, right? > > > > Yeah, the dequeue case makes we have to care about pick, not sure we > > then also need to care about sched_update_tick_dependency() though. > > There is indeed a window where these two will 'race', but afaict it is > > benign. > > > > Hm, that's confusing. > > As I see it it's the enqueue case (0->1 mostly) where we need the check > in pick. At that point in enqueue we only have a handle on ->curr which > is the idle thread.
Well, the 0->1 case is trivial, we'll run the task that's enqueued, and as such everything can DTRT and be simple.
> For the dequeue case (2->1) we need the check in the > sched_update_tick_dependency() path because if the 1 is the task on the > cpu (and is staying there) then we'd otherwise clear the bit when we > shouldn't (since we aren't going to go back through pick).
The 2->1 case OTOH is tricky, because then we'll end up running a task we've not recently seen. sub_nr_running() will hit the ==1 case and clear TICK_DEP_BIT_SCHED.
But then pick will come and set it again, no harm done, right?
.oO Ah!, You're worried about the case where a task is already running, a second task comes in, (1->2) and then quickly leaves again (2->1) without passing through schedule(). And you don't want to disable the tick if that running task needs it.
Mooo :-(
> I'm thinking that I'll try to set the bit in pick since we only care about > it when it's the task on the cpu. That, I think, will simplify the > code needed to update the bit when the quota is changed (to or from > RUNTIME_INF). > > Setting the bit in enqueue/dequeue means updating it on all the queued > task if it changes. Although I may clear it in dequeue just to not leave > it around stale.
Hmm, no you have to set on enqueue (1->2), otherwise the running task doesn't get preempted when it runs out of slice.
And I don't suppose you want to delay clearing to the first tick after, because NOHZ_FULL doesn't want spurious ticks :/
What a mess.
Please document all these stupid cases in a comment, otherwise we'll go bananas trying to make sense of the code later on.
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