Re: [PATCH RESEND] sched/nohz: Affine unpinned timers to housekeepers

[Frederic Weisbecker <fweisbec@gmail.com>]

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 <fweisbec@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 <fweisbec@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