* 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?
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.
Thanks,
Ingo
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