linux-kernel - Re: [PATCH] time/tick-broadcast: Fix tick_broadcast

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Date:   Fri, 21 Jun 2019 10:50:27 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        frederic@...nel.org, tglx@...utronix.de
Subject: Re: [PATCH] time/tick-broadcast: Fix tick_broadcast_offline()
 lockdep complaint

On Fri, Jun 21, 2019 at 10:41:04AM -0700, Paul E. McKenney wrote:
> On Fri, Jun 21, 2019 at 06:34:14AM -0700, Paul E. McKenney wrote:
> > On Fri, Jun 21, 2019 at 02:29:27PM +0200, Peter Zijlstra wrote:
> > > On Fri, Jun 21, 2019 at 05:16:30AM -0700, Paul E. McKenney wrote:
> > > > A pair of full hangs at boot (TASKS03 and TREE04), no console output
> > > > whatsoever.  Not sure how these changes could cause that, but suspicion
> > > > falls on sched_tick_offload_init().  Though even that is a bit strange
> > > > because if so, why didn't TREE01 and TREE07 also hang?  Again, looking
> > > > into it.
> > > 
> > > Pesky details ;-)
> > 
> > And backing out to the earlier patch removes the hangs, though statistical
> > insignificance and all that.
> 
> And purists might argue that four failures out of four attempts does not
> constitute true statistical significance, but too bad.  If I interpose
> a twork pointer in sched_tick_offload_init()'s initialization, it seems
> to work fine, give or take lack of statistical significance.  This is
> surprising, so I am rerunning with added parentheses in the atomic_set()
> expression.

Huh.  This works, albeit only once:

	int __init sched_tick_offload_init(void)
	{
		struct tick_work *twork;
		int cpu;

		tick_work_cpu = alloc_percpu(struct tick_work);
		BUG_ON(!tick_work_cpu);
		for_each_possible_cpu(cpu) {
			twork = per_cpu_ptr(tick_work_cpu, cpu);
			atomic_set(&twork->state, TICK_SCHED_REMOTE_OFFLINE);
		}

		return 0;
	}

This does not work:

	int __init sched_tick_offload_init(void)
	{
		int cpu;

		tick_work_cpu = alloc_percpu(struct tick_work);
		BUG_ON(!tick_work_cpu);
		for_each_possible_cpu(cpu)
			atomic_set(&(per_cpu(tick_work_cpu, cpu)->state), TICK_SCHED_REMOTE_OFFLINE);

		return 0;
	}

I will run more tests on the one that worked only once.  In the meantime,
feel free to tell me what stupid thing I did with the parentheses.

								Thanx, Paul