On Mon, Aug 24, 2015 at 04:04:37PM +0200, Frederic Weisbecker wrote: > On Mon, Aug 24, 2015 at 06:50:18AM -0700, Paul E. McKenney wrote: > > 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. > > Well nohz full is likely to be used for HPC and that can involve big machines. > Having the housekeeping duty spread per node is a likely future evolution > there, > if it isn't already used that way. > > So we need to keep it a cpumask.
Fair point! Thanx, Paul > > > 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. > > Indeed, more specifically tick-common.c makes sure to set the timekeeping > duty to a housekeeper and that housekeeper is always the boot CPU due to > early device initialization. > > But I should find a way to simplify that code and make it obvious it's always > set to the boot CPU. > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/