linux-kernel - Re: [PATCH v4 14/16] timer: Implement the hierarchical pull model

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Date:   Tue, 29 Nov 2022 11:30:21 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Anna-Maria Behnsen <anna-maria@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        John Stultz <jstultz@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Eric Dumazet <edumazet@...gle.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Arjan van de Ven <arjan@...radead.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH v4 14/16] timer: Implement the hierarchical pull model

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.