Re: [PATCH RESEND] sched/nohz: Affine unpinned timers to housekeepers

[Paul E. McKenney]

On Mon, Aug 24, 2015 at 08:44:12AM +0200, Ingo Molnar wrote:
> 
> * Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> 
> > > here it's fully set - triggering the bug I'm worried about. So what am I 
> > > missing, what prevents CONFIG_NO_HZ_FULL_ALL from crashing?
> > 
> > The boot CPU is excluded from tick_nohz_full_mask in tick_nohz_init(), which is 
> > called from tick_init() which is called from start_kernel() shortly after 
> > rcu_init():
> > 
> > 	cpu = smp_processor_id();
> > 
> > 	if (cpumask_test_cpu(cpu, tick_nohz_full_mask)) {
> > 		pr_warning("NO_HZ: Clearing %d from nohz_full range for timekeeping\n", cpu);
> > 		cpumask_clear_cpu(cpu, tick_nohz_full_mask);
> > 	}
> > 
> > This happens after the call to tick_nohz_init_all() that does the 
> > cpumask_setall() that you called out above.
> 
> Ah, indeed - I somehow missed that.
> 
> This brings up two other questions:
> 
> 1)
> 
> the 'housekeeping CPU' is essentially the boot CPU. Yet we dedicate a full mask to 
> it (housekeeping_mask - a variable mask to begin with) and recover the 
> housekeeping CPU via:
> 
> +       return cpumask_any_and(housekeeping_mask, cpu_online_mask);
> 
> which can be pretty expensive, and which gets executed in two hotpaths:
> 
> kernel/time/hrtimer.c:  return &per_cpu(hrtimer_bases, get_nohz_timer_target());
> kernel/time/timer.c:    return per_cpu_ptr(&tvec_bases, get_nohz_timer_target());
> 
> ... why not just use a single housekeeping_cpu which would be way faster to pass 
> down to the timer code?

The housekeeping_cpu came later, but that does seem like a good optimization.

> 2)
> 
> What happens if the boot CPU is offlined? (under CONFIG_BOOTPARAM_HOTPLUG_CPU0=y)
> 
> I don't see CPU hotplug callbacks fixing up the housekeeping_mask if the boot CPU 
> is offlined.

The tick_nohz_cpu_down_callback() function does this, though in a less
than obvious way.  The tick_do_timer_cpu variable is the housekeeping
CPU that is currently handling timing, and it is not permitted to go
offline.

							Thanx, Paul

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