Messages in this thread | | | Date | Tue, 29 Nov 2022 11:30:21 +0100 | From | Frederic Weisbecker <> | Subject | Re: [PATCH v4 14/16] timer: Implement the hierarchical pull model |
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On Mon, Nov 28, 2022 at 05:20:10PM +0100, Frederic Weisbecker wrote: > On Thu, Nov 24, 2022 at 08:47:56AM +0100, Anna-Maria Behnsen wrote: > > On Tue, 15 Nov 2022, Frederic Weisbecker wrote: > > > > > On Fri, Nov 04, 2022 at 03:57:35PM +0100, Anna-Maria Behnsen wrote: > > > > +static int tmigr_cpu_offline(unsigned int cpu) > > > > +{ > > > > + struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu); > > > > + > > > > + raw_spin_lock_irq(&tmc->lock); > > > > + tmc->online = 0; > > > > + __tmigr_cpu_deactivate(tmc, KTIME_MAX); > > > > > > This means that if the CPU is going idle for some time during > > > the hotplug process (ie: at some point between CPUHP_AP_TMIGR_ONLINE > > > and CPUHP_TEARDOWN_CPU), then a global timer may be delayed for that long. > > > > > > I guess it shouldn't be too bad but worth mentioning... > > > > > > Although if it happens to be a problem it could be solved with simply allowing > > > tmigr_cpu_deactivate() when !tmc->online. > > > > The plan was (and I broke it) to let the CPU handle global timers by itself > > as long as timer migration hierarchy is not completely initialized and as > > long as CPU is marked offline in timer migration hierarchy. Otherwise > > global timers might be delayed during this period. The proper way would be > > that tmigr_cpu_deactivate(nextexp) directly returns nextexp if !tmc->online > > and tmigr hierarchy is not in place yet. I will have a deeper look if there > > was a reason why I changed the return to KTIME_MAX... > > Well, do you need tmc->online at all? > > I guess tmigr_cpu_offline() could be simply removed because when the CPU goes > finally down in do_idle() it does: > > tick_nohz_idle_stop_tick(); > cpuhp_report_idle_dead(); > > So the tick is expected to be stopped (better check it is) and thus the tmc > should be deactivated. > > I guess you just need to call tmigr_cpu_activate() from timers_prepare_cpu().
Now of course the issue is that between CPUHP_TIMERS_PREPARE and CPUHP_AP_ONLINE_IDLE the tglobal timers may not be served.
One way to deal with that is to call tmigr_cpu_activate() from cpu_startup_entry().
Otherwise then back to tmc->online I guess.
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