On Sun, Aug 23, 2015 at 07:40:32AM +0200, Ingo Molnar wrote: > So I almost applied this yesterday, but had the following question: what > ensures > that housekeeping_mask isn't empty? If it's empty then housekeeping_any_cpu() > returns cpumask_any_and() of an empty cpumask - which returns an out of range > index AFAICS - which will crash and burn in: > > 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()); > > housekeeping_mask itself is derived from tick_nohz_full_mask (it's the > inverse of > it in essence), and tick_nohz_full_mask is set via two methods, either via a > boot > parameter: > > if (cpulist_parse(str, tick_nohz_full_mask) < 0) { > > in tick_nohz_full_setup(). What ensures here that tick_nohz_full_mask is not > completely full - making housekeeping_mask empty? > > The other method is via CONFIG_NO_HZ_FULL_ALL: > > cpumask_setall(tick_nohz_full_mask); > > 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?
Legitimate worry and I should have explained that in the changelog. Like Paul replied, we make sure that at least the boot CPU is excluded from tick_nohz_full_mask in tick_nohz_init(). Then housekeeping_mask, by reverse effect, contains that boot CPU at least. And we also make sure that the boot CPU can't get offline (tick_nohz_cpu_down_callback()). Now we should really document and check that assumption so here is a second patch below. The sched patch depends on tip:sched/core (to avoid conflicts with sched changes) and the following one is based on tip:timer/nohz but should be applicable to sched/core without conflict. Both are standalone anyway. Thanks! --- From: Frederic Weisbecker <fweis...@gmail.com> Date: Sun, 23 Aug 2015 19:34:31 +0200 Subject: [PATCH] nohz: Assert existing housekeepers when nohz full enabled The code ensures that at least the boot CPU serves as a housekeeper. Let's assert this assumption to make sure that we have CPUs to handle unbound jobs like workqueues and timers while nohz full CPUs run undisturbed. Signed-off-by: Frederic Weisbecker <fweis...@gmail.com> --- kernel/time/tick-sched.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index 3319e16..cc9884f 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -370,6 +370,12 @@ void __init tick_nohz_init(void) cpu_notifier(tick_nohz_cpu_down_callback, 0); pr_info("NO_HZ: Full dynticks CPUs: %*pbl.\n", cpumask_pr_args(tick_nohz_full_mask)); + + /* + * We need at least one CPU to handle housekeeping work such + * as timekeeping, unbound timers, workqueues, ... + */ + WARN_ON_ONCE(cpumask_empty(housekeeping_mask)); } #endif -- 2.1.4 -- 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/