Subject: + kernel-watchdogc-touch_nmi_watchdog-should-only-touch-local-cpu-not-every-one.patch added to -mm tree To: benzh@xxxxxxxxxxxx,dzickus@xxxxxxxxxx From: akpm@xxxxxxxxxxxxxxxxxxxx Date: Mon, 13 Jan 2014 12:03:13 -0800 The patch titled Subject: kernel/watchdog.c: touch_nmi_watchdog should only touch local cpu not every one has been added to the -mm tree. Its filename is kernel-watchdogc-touch_nmi_watchdog-should-only-touch-local-cpu-not-every-one.patch This patch should soon appear at http://ozlabs.org/~akpm/mmots/broken-out/kernel-watchdogc-touch_nmi_watchdog-should-only-touch-local-cpu-not-every-one.patch and later at http://ozlabs.org/~akpm/mmotm/broken-out/kernel-watchdogc-touch_nmi_watchdog-should-only-touch-local-cpu-not-every-one.patch Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/SubmitChecklist when testing your code *** The -mm tree is included into linux-next and is updated there every 3-4 working days ------------------------------------------------------ From: Ben Zhang <benzh@xxxxxxxxxxxx> Subject: kernel/watchdog.c: touch_nmi_watchdog should only touch local cpu not every one I ran into a scenario where while one cpu was stuck and should have panic'd because of the NMI watchdog, it didn't. The reason was another cpu was spewing stack dumps on to the console. Upon investigation, I noticed that when writing to the console and also when dumping the stack, the watchdog is touched. This causes all the cpus to reset their NMI watchdog flags and the 'stuck' cpu just spins forever. This change causes the semantics of touch_nmi_watchdog to be changed slightly. Previously, I accidentally changed the semantics and we noticed there was a codepath in which touch_nmi_watchdog could be touched from a preemtible area. That caused a BUG() to happen when CONFIG_DEBUG_PREEMPT was enabled. I believe it was the acpi code. My attempt here re-introduces the change to have the touch_nmi_watchdog() code only touch the local cpu instead of all of the cpus. But instead of using __get_cpu_var(), I use the __raw_get_cpu_var() version. This avoids the preemption problem. However my reasoning wasn't because I was trying to be lazy. Instead I rationalized it as, well if preemption is enabled then interrupts should be enabled to and the NMI watchdog will have no reason to trigger. So it won't matter if the wrong cpu is touched because the percpu interrupt counters the NMI watchdog uses should still be incrementing. Don said: : I'm ok with this patch, though it does alter the behaviour of how : touch_nmi_watchdog works. For the most part I don't think most callers : need to touch all of the watchdogs (on each cpu). Perhaps a corner case : will pop up (the scheduler?? to mimic touch_all_softlockup_watchdogs() ). : : But this does address an issue where if a system is locked up and one cpu : is spewing out useful debug messages (or error messages), the hard lockup : will fail to go off. We have seen this on RHEL also. Signed-off-by: Don Zickus <dzickus@xxxxxxxxxx> Signed-off-by: Ben Zhang <benzh@xxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- kernel/watchdog.c | 16 ++++++++-------- 1 file changed, 8 insertions(+), 8 deletions(-) diff -puN kernel/watchdog.c~kernel-watchdogc-touch_nmi_watchdog-should-only-touch-local-cpu-not-every-one kernel/watchdog.c --- a/kernel/watchdog.c~kernel-watchdogc-touch_nmi_watchdog-should-only-touch-local-cpu-not-every-one +++ a/kernel/watchdog.c @@ -158,14 +158,14 @@ void touch_all_softlockup_watchdogs(void #ifdef CONFIG_HARDLOCKUP_DETECTOR void touch_nmi_watchdog(void) { - if (watchdog_user_enabled) { - unsigned cpu; - - for_each_present_cpu(cpu) { - if (per_cpu(watchdog_nmi_touch, cpu) != true) - per_cpu(watchdog_nmi_touch, cpu) = true; - } - } + /* + * Using __raw here because some code paths have + * preemption enabled. If preemption is enabled + * then interrupts should be enabled too, in which + * case we shouldn't have to worry about the watchdog + * going off. + */ + __raw_get_cpu_var(watchdog_nmi_touch) = true; touch_softlockup_watchdog(); } EXPORT_SYMBOL(touch_nmi_watchdog); _ Patches currently in -mm which might be from benzh@xxxxxxxxxxxx are kernel-watchdogc-touch_nmi_watchdog-should-only-touch-local-cpu-not-every-one.patch -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html