Messages in this thread | | | Date | Sat, 6 Aug 2011 10:20:56 +1000 | From | Dave Chinner <> | Subject | trigger_all_cpu_backtrace() has no generic implementation (was Re: [regression, 3.1, rcu] rcu_sched_state detected stall on CPU 8 (t=15000 jiffies)) |
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On Fri, Aug 05, 2011 at 04:24:29AM -0700, Paul E. McKenney wrote: > On Fri, Aug 05, 2011 at 06:48:39PM +1000, Dave Chinner wrote: > > On Thu, Aug 04, 2011 at 11:41:19PM -0700, Paul E. McKenney wrote: > > > On Fri, Aug 05, 2011 at 10:33:16AM +1000, Dave Chinner wrote: > > > > On Tue, Aug 02, 2011 at 11:30:50PM -0700, Paul E. McKenney wrote: > > > > > On Wed, Aug 03, 2011 at 12:52:22PM +1000, Dave Chinner wrote: > > > > > > On Wed, Aug 03, 2011 at 12:28:57PM +1000, Dave Chinner wrote: > > > > > > > Hi Paul, > > > > > > > > > > > > > > I've had this hang a couple of times now, so I figured it isn't an > > > > > > > isolated event. I am getting kernels occassionally hanging with the > > > > > > > following output occurring: > > > > > > > > > > > > > > [ 62.812011] INFO: rcu_sched_state detected stall on CPU 8 (t=15000 jiffies) > > > > > > > [ 242.936009] INFO: rcu_sched_state detected stall on CPU 8 (t=60031 jiffies) > > > > > > > > .... > > > > > > > > > > This might be a false alarm - I've just diagnosed(*) that a kernel > > > > > > thread was stuck in a hard loop therefore not giving up the CPU. > > > > > > > > > > Ah, that is indeed one of the conditions that RCU CPU stall warnings > > > > > can catch. > > > > > > > > > > > Perhaps this is error message could be more informative? > > > > > > The detector is acting like the hung task detector, except it's > > > > > > working on kernel code stuck in a loop burning CPU, so maybe dumping > > > > > > a stack trace of the spinning CPU (i.e. similar to sysrq-l output) > > > > > > might be a useful addition to tracking down such stalls? > > > > > > > > > > Strange. There is a trigger_all_cpu_backtrace() call that is supposed > > > > > to dump all CPUs' stacks. It has been working in the past, but you are > > > > > the second person in a couple of weeks to report that it isn't doing > > > > > its job. (Though the other one was running the -rt tree.) > > > > > > > > Ok, so it is supposed to be dumping the stack. Good. > > > > > > Yep! .... > > > > I'm running on x86_64 (inside a KVM VM) so it should be present. > > > > > > Indeed it should! Is NMI delivery busted or something? > > > > Not that I know of. .... > > > Just out of curiosity, what are you thinking of doing in the code to > > > figure out that trigger_all_cpu_backtrace() didn't work and that it > > > was time to fall back on sysrq-l-style processing? > > > > - trigger_all_cpu_backtrace(); > > + if (!trigger_all_cpu_backtrace()) { > > + pr_err("trigger_all_cpu_backtrace returned false!"); > > So a good defensive-programming patch would be something like the > following, right? > > if (!trigger_all_cpu_backtrace()) { > pr_err("Falling back to dump_stack()"); > dump_stack(); > } > > > And I see: > > > > [ 90.808037] INFO: rcu_sched_state detected stall on CPU 8 (t=15000 jiffies) > > [ 90.808037] trigger_all_cpu_backtrace returned false > > > > Which indicates that arch_trigger_all_cpu_backtrace is indeed not > > defined, and that sysrq-l is using the fallback. > > > > My .config is below if you want to look into it further. > > My ability to do so is quite limited, but you do have > CONFIG_X86_LOCAL_APIC=y. I took a quick look but don't immediately > see why you would not be getting the NMI variant.
It would appear to be because include/linux/nmi.h does not do #include <asm/nmi.h> where arch_trigger_all_cpu_backtrace is defined.
I think this is the case from looking at the build deps for rcutree.h: I see include/linux/nmi.h but no asm/nmi.h. It appears that asm/nmi.h is only included if this config ifdef is true:
#if defined(ARCH_HAS_NMI_WATCHDOG) || defined(CONFIG_HARDLOCKUP_DETECTOR) #include <asm/nmi.h>
And my .config has:
$ grep CONFIG_HARDLOCKUP_DETECTOR .config # CONFIG_HARDLOCKUP_DETECTOR is not set $
And ARCH_HAS_NMI_WATCHDOG is only defined for these archs:
$ grep ARCH_HAS_NMI_WATCHDOG -r arch arch/blackfin/include/asm/irq.h:# define ARCH_HAS_NMI_WATCHDOG arch/mn10300/include/asm/reset-regs.h:#define ARCH_HAS_NMI_WATCHDOG /* See include/linux/nmi.h */ arch/sparc/include/asm/irq_64.h:#define ARCH_HAS_NMI_WATCHDOG $
So unless you configure the CONFIG_HARDLOCKUP_DETECTOR into a kernel, trigger_all_cpu_backtrace() will simply fail.
And i can't just convert this to always include asm/nmi.h, because:
$ find arch -name nmi.h arch/blackfin/include/asm/nmi.h arch/mips/include/asm/sn/nmi.h arch/mn10300/include/asm/nmi.h arch/s390/include/asm/nmi.h arch/sparc/include/asm/nmi.h arch/x86/include/asm/nmi.h $
Only a few platforms actually define asm/nmi.h. The whole platform specific trigger_all_cpu_backtrace() stuff is just broken - it's not just platform specific, it's platform and config specific, without having any indication that it is config specific....
Arch specific code is supposed to have a generic implementation for platfoms that don't implement it. It's pretty clear from the sysrq-l code that a generic implementation is possible, so I'm wondering why trigger_all_cpu_backtrace doesn't own that generic fallback code.....
Cheers,
Dave. -- Dave Chinner david@fromorbit.com
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