Re: [patch 2/3] nohz: fix printk_needs_cpu() return value on offline cpus

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Fri, 2010-11-26 at 13:00 +0100, Heiko Carstens wrote:
> plain text document attachment (002_printk_needs_cpu.diff)
> From: Heiko Carstens <heiko.carstens@de.ibm.com>
> 
> This patch fixes a hang observed with 2.6.32 kernels where timers got
> enqueued on offline cpus.
> 
> printk_needs_cpu() may return 1 if called on offline cpus. When a cpu gets
> offlined it schedules the idle process which, before killing its own cpu,
> will call tick_nohz_stop_sched_tick().
> That function in turn will call printk_needs_cpu() in order to check if the
> local tick can be disabled. On offline cpus this function should naturally
> return 0 since regardless if the tick gets disabled or not the cpu will be
> dead short after. That is besides the fact that __cpu_disable() should already
> have made sure that no interrupts on the offlined cpu will be delivered anyway.
> 
> In this case it prevents tick_nohz_stop_sched_tick() to call
> select_nohz_load_balancer(). No idea if that really is a problem. However what
> made me debug this is that on 2.6.32 the function get_nohz_load_balancer() is
> used within __mod_timer() to select a cpu on which a timer gets enqueued.
> If printk_needs_cpu() returns 1 then the nohz_load_balancer cpu doesn't get
> updated when a cpu gets offlined. It may contain the cpu number of an offline
> cpu. In turn timers get enqueued on an offline cpu and not very surprisingly
> they never expire and cause system hangs.
> 
> This has been observed 2.6.32 kernels. On current kernels __mod_timer() uses
> get_nohz_timer_target() which doesn't have that problem. However there might
> be other problems because of the too early exit tick_nohz_stop_sched_tick()
> in case a cpu goes offline.
> 
> Easiest way to fix this is just to test if the current cpu is offline and
> call printk_tick() directly which clears the condition.
> 
> Alternatively I tried a cpu hotplug notifier which would clear the condition,
> however between calling the notifier function and printk_needs_cpu() something
> could have called printk() again and the problem is back again. This seems to
> be the safest fix.
> 
> Cc: stable@kernel.org
> Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
> ---
>  kernel/printk.c |    2 ++
>  1 file changed, 2 insertions(+)
> 
> --- a/kernel/printk.c
> +++ b/kernel/printk.c
> @@ -1082,6 +1082,8 @@ void printk_tick(void)
>  
>  int printk_needs_cpu(int cpu)
>  {
> +	if (unlikely(cpu_is_offline(cpu)))
> +		printk_tick();
>  	return per_cpu(printk_pending, cpu);
>  }
>  

Nice,.. applied.