Messages in this thread | | | Date | Tue, 7 Dec 2010 13:32:41 -0800 | From | Greg KH <> | Subject | Re: [stable] [patch 2/3] nohz: fix printk_needs_cpu() return value on offline cpus |
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On Fri, Nov 26, 2010 at 01:11:52PM +0100, Peter Zijlstra wrote: > 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.
Is this going to make it into .37, or is it going to wait until .38?
thanks,
greg k-h
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