Messages in this thread Patch in this message | | | Date | Mon, 8 Dec 2014 08:54:05 -0500 | From | Steven Rostedt <> | Subject | Re: [PATCH] kthread: kthread_bind fails to enforce CPU affinity (fixes kernel BUG at kernel/smpboot.c:134!) |
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On Mon, 8 Dec 2014 14:27:01 +1100 Anton Blanchard <anton@samba.org> wrote:
> I have a busy ppc64le KVM box where guests sometimes hit the infamous > "kernel BUG at kernel/smpboot.c:134!" issue during boot: > > BUG_ON(td->cpu != smp_processor_id()); > > Basically a per CPU hotplug thread scheduled on the wrong CPU. The oops > output confirms it: > > CPU: 0 > Comm: watchdog/130 > > The issue is in kthread_bind where we set the cpus_allowed mask, but do > not touch task_thread_info(p)->cpu. The scheduler assumes the previously > scheduled CPU is in the cpus_allowed mask, but in this case we are > moving a thread to another CPU so it is not. >
Does this happen always on boot up, and always with the watchdog thread?
I followed the logic that starts the watchdog threads.
watchdog_enable_all_cpus() smpboot_register_percpu-thread() {
for_each_online_cpu(cpu) { ... }
Where watchdog_enable_all_cpus() can be called by lockup_detector_init() before SMP is started, but also by proc_dowatchdog() which is called by the sysctl commands (after SMP is up and running).
I noticed there's no "get_online_cpus()" anywhere, although the unregister_percpu_thread() has it. Is it possible that we created a thread on a CPU that wasn't fully online yet?
Perhaps the following patch is needed? Even if this isn't the solution to this bug, it is probably needed as watchdog_enable_all_cpus() can be called after boot up too.
-- Steve
diff --git a/kernel/smpboot.c b/kernel/smpboot.c index eb89e1807408..60d35ac5d3f1 100644 --- a/kernel/smpboot.c +++ b/kernel/smpboot.c @@ -279,6 +279,7 @@ int smpboot_register_percpu_thread(struct smp_hotplug_thread *plug_thread) unsigned int cpu; int ret = 0; + get_online_cpus(); mutex_lock(&smpboot_threads_lock); for_each_online_cpu(cpu) { ret = __smpboot_create_thread(plug_thread, cpu); @@ -291,6 +292,7 @@ int smpboot_register_percpu_thread(struct smp_hotplug_thread *plug_thread) list_add(&plug_thread->list, &hotplug_threads); out: mutex_unlock(&smpboot_threads_lock); + put_online_cpus(); return ret; } EXPORT_SYMBOL_GPL(smpboot_register_percpu_thread);
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