Messages in this thread | | | Date | Mon, 24 Aug 2015 06:50:18 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RESEND] sched/nohz: Affine unpinned timers to housekeepers |
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On Mon, Aug 24, 2015 at 08:44:12AM +0200, Ingo Molnar wrote: > > * 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?
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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