Messages in this thread | | | Date | Mon, 24 Aug 2015 08:44:12 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH RESEND] sched/nohz: Affine unpinned timers to housekeepers |
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* Paul E. McKenney <paulmck@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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