On Mon, Aug 24, 2015 at 08:44:12AM +0200, Ingo Molnar wrote:
>
> * Paul E. McKenney <paul...@linux.vnet.ibm.com> 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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