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Date: Thu, 4 Aug 2011 23:41:19 -0700 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Dave Chinner <david@...morbit.com> Cc: linux-kernel@...r.kernel.org Subject: Re: [regression, 3.1, rcu] rcu_sched_state detected stall on CPU 8 (t=15000 jiffies) 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! > > Wait a minute... Here is the definition: > > > > #ifdef arch_trigger_all_cpu_backtrace > > static inline bool trigger_all_cpu_backtrace(void) > > { > > arch_trigger_all_cpu_backtrace(); > > > > return true; > > } > > #else > > static inline bool trigger_all_cpu_backtrace(void) > > { > > return false; > > } > > #endif > > > > Passing a lower-case symbol to #ifdef is a bit of a red flag. Where > > is it defined? > > > > o arch/sparc/include/asm/irq_64.h: > > > > #define arch_trigger_all_cpu_backtrace arch_trigger_all_cpu_backtrace > > > > o arch/sparc/kernel/process_64.c: > > > > void arch_trigger_all_cpu_backtrace(void) > > { > > ... > > } > > > > o arch/x86/include/asm/nmi.h: > > > > #define arch_trigger_all_cpu_backtrace arch_trigger_all_cpu_backtrace > > > > o arch/x86/kernel/apic/hw_nmi.c: > > > > void arch_trigger_all_cpu_backtrace(void) > > { > > ... > > } > > > > So I am guessing that you are running some architecture other than > > x86 or SPARC. And the implementation is a bit hostile on other > > architectures. So I suggest adding a dump_stack() before the > > "return false" in trigger_all_cpu_backtrace(), as in the patch > > shown below. > > I'm running on x86_64 (inside a KVM VM) so it should be present. Indeed it should! Is NMI delivery busted or something? In addition, how about putting a compiler error into trigger_all_cpu_backtrace() and verifying that your build fails? Just in case and all that. > Hmmm - I note that sysrq-l has a fallback implementation that uses > smp_call_function() should trigger_all_cpu_backtrace() return false. > I'd bet that's why sysrq-l is working and the rcu stall detection > isn't. i.e arch_trigger_all_cpu_backtrace() is either broken or for > some reason not compiled in. I can't tell why - I get lost in all > the different ways that arch specific code is inlined by > preprocessor magic... Interesting. > > But this is still strange. I -know- I have seen stack dumps for > > all CPUs when running on Power... But the code has not changed > > for quite some time. > > > > Nevertheless, could you please try out the patch below? It should > > get you at least the stack dump for the current CPU, which in your > > case was the offending CPU. > > I'll give it a go, though perhaps using the same fallback as sysrq-l > might be a better idea? If your architecture really does believe that trigger_all_cpu_backtrace() is available, my patch will have no effect. Substituting dump_stack() for trigger_all_cpu_backtrace() would be helpful. 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? Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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