Messages in this thread | | | Date | Mon, 17 Apr 2023 15:28:30 +0200 | From | Frederic Weisbecker <> | Subject | Re: [PATCH v2] tick/broadcast: Do not set oneshot_mask except was_periodic was true |
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Le Sat, Apr 15, 2023 at 11:01:51PM +0200, Thomas Gleixner a écrit : > > * CPU 1 stop its tick, next event is in one hour. It calls > > tick_broadcast_enter() and goes to sleep. > > So there is already a broadcast device installed, right?
Yes
> > > * CPU 1 gets an interrupt that enqueues a new timer expiring in the next jiffy > > (note it's not yet actually programmed in the tick device) > > * CPU 1 call tick_broadcast_exit(). > > * CPU 0 registers new broadcast device and sets CPU 1 in tick_broadcast_oneshot_mask > > This lacks an explanation why CPU0 sets CPU1 in that mask. It does not > _set_ it explicitely, only implicitely by ORing the periodic broadcast > cpumask over. > > Now the question is why is CPU1 set in the periodic broadcast mask when > the CPU already switched over to NOHZ mode? > > That needs to be explained too.
I probably got confused with that tick_broadcast_mask, so it's only set for periodic broadcast? Should it be renamed to tick_periodic_broadcast_mask to disambiguate my eternal confusion?
> > > * CPU 0 runs the broadcast callback, sees that the next timer for CPU 1 > > is in one hour (because the recently enqueued timer for CPU 1 hasn't been programmed > > yet), so it programs the broadcast to that 1 hour deadline. > > * CPU 1 runs tick_nohz_idle_stop_tick() which eventually writes and program > > dev->next_event to next jiffy > > * CPU 1 runs into cpuidle_enter_state(), and tick_broadcast_enter() is ignored because > > the CPU is already in tick_broadcast_oneshot_mask, so the dev->next_event > > change isn't propagated to broadcast. > > * CPU 1 goes to sleep for 1 hour. > > Also please use tabular style to explain the parallel events as > explained in the documentation.
Yeah my bad I asked Victor to integrate that scenario that popped out of me misunderstanding that code. Not even mentioning the form.
Now to review your proposal.
Thanks.
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