debug lockups: Improve lockup detection

When debugging a recent lockup bug i found various deficiencies
in how our current lockup detection helpers work:

 - SysRq-L is not very efficient as it uses a workqueue, hence
   it cannot punch through hard lockups and cannot see through
   most soft lockups either.

 - The SysRq-L code depends on the NMI watchdog - which is off
   by default.

 - We dont print backtraces from the RCU code's built-in
   'RCU state machine is stuck' debug code. This debug
   code tends to be one of the first (and only) mechanisms
   that show that a lockup has occured.

This patch changes the code so taht we:

 - Trigger the NMI backtrace code from SysRq-L instead of using
   a workqueue (which cannot punch through hard lockups)

 - Trigger print-all-CPU-backtraces from the RCU lockup detection
   code

Also decouple the backtrace printing code from the NMI watchdog:

 - Dont use variable size cpumasks (it might not be initialized
   and they are a bit more fragile anyway)

 - Trigger an NMI immediately via an IPI, instead of waiting
   for the NMI tick to occur. This is a lot faster and can
   produce more relevant backtraces. It will also work if the
   NMI watchdog is disabled.

 - Dont print the 'dazed and confused' message when we print
   a backtrace from the NMI

 - Do a show_regs() plus a dump_stack() to get maximum info
   out of the dump. Worst-case we get two stacktraces - which
   is not a big deal. Sometimes, if register content is
   corrupted, the precise stack walker in show_regs() wont
   give us a full backtrace - in this case dump_stack() will
   do it.

Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

3 files changed, 20 insertions, 13 deletions

diff --git a/arch/x86/kernel/apic/nmi.c b/arch/x86/kernel/apic/nmi.c
index b3025b43b63a..1bb1ac20e9ec 100644
--- a/arch/x86/kernel/apic/nmi.c
+++ b/arch/x86/kernel/apic/nmi.c
@@ -39,7 +39,7 @@
 int unknown_nmi_panic;
 int nmi_watchdog_enabled;
 
-static cpumask_var_t backtrace_mask;
+static cpumask_t backtrace_mask __read_mostly;
 
 /* nmi_active:
  * >0: the lapic NMI watchdog is active, but can be disabled
@@ -138,7 +138,6 @@ int __init check_nmi_watchdog(void)
         if (!prev_nmi_count)
                 goto error;
 
-        alloc_cpumask_var(&backtrace_mask, GFP_KERNEL|__GFP_ZERO);
         printk(KERN_INFO "Testing NMI watchdog ... ");
 
 #ifdef CONFIG_SMP
@@ -415,14 +414,17 @@ nmi_watchdog_tick(struct pt_regs *regs, unsigned reason)
         }
 
         /* We can be called before check_nmi_watchdog, hence NULL check. */
-        if (backtrace_mask != NULL && cpumask_test_cpu(cpu, backtrace_mask)) {
+        if (cpumask_test_cpu(cpu, &backtrace_mask)) {
                 static DEFINE_SPINLOCK(lock);   /* Serialise the printks */
 
                 spin_lock(&lock);
                 printk(KERN_WARNING "NMI backtrace for cpu %d\n", cpu);
+                show_regs(regs);
                 dump_stack();
                 spin_unlock(&lock);
-                cpumask_clear_cpu(cpu, backtrace_mask);
+                cpumask_clear_cpu(cpu, &backtrace_mask);
+
+                rc = 1;
         }
 
         /* Could check oops_in_progress here too, but it's safer not to */
@@ -556,10 +558,14 @@ void __trigger_all_cpu_backtrace(void)
 {
         int i;
 
-        cpumask_copy(backtrace_mask, cpu_online_mask);
+        cpumask_copy(&backtrace_mask, cpu_online_mask);
+
+        printk(KERN_INFO "sending NMI to all CPUs:\n");
+        apic->send_IPI_all(NMI_VECTOR);
+
         /* Wait for up to 10 seconds for all CPUs to do the backtrace */
         for (i = 0; i < 10 * 1000; i++) {
-                if (cpumask_empty(backtrace_mask))
+                if (cpumask_empty(&backtrace_mask))
                         break;
                 mdelay(1);
         }
diff --git a/drivers/char/sysrq.c b/drivers/char/sysrq.c
index 5d7a02f63e1c..165f307f30e8 100644
--- a/drivers/char/sysrq.c
+++ b/drivers/char/sysrq.c
@@ -24,6 +24,7 @@
 #include <linux/sysrq.h>
 #include <linux/kbd_kern.h>
 #include <linux/proc_fs.h>
+#include <linux/nmi.h>
 #include <linux/quotaops.h>
 #include <linux/perf_counter.h>
 #include <linux/kernel.h>
@@ -222,12 +223,7 @@ static DECLARE_WORK(sysrq_showallcpus, sysrq_showregs_othercpus);
 
 static void sysrq_handle_showallcpus(int key, struct tty_struct *tty)
 {
-        struct pt_regs *regs = get_irq_regs();
-        if (regs) {
-                printk(KERN_INFO "CPU%d:\n", smp_processor_id());
-                show_regs(regs);
-        }
-        schedule_work(&sysrq_showallcpus);
+        trigger_all_cpu_backtrace();
 }
 
 static struct sysrq_key_op sysrq_showallcpus_op = {
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 7717b95c2027..9c5fa9fc57ec 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -35,6 +35,7 @@
 #include <linux/rcupdate.h>
 #include <linux/interrupt.h>
 #include <linux/sched.h>
+#include <linux/nmi.h>
 #include <asm/atomic.h>
 #include <linux/bitops.h>
 #include <linux/module.h>
@@ -469,6 +470,8 @@ static void print_other_cpu_stall(struct rcu_state *rsp)
         }
         printk(" (detected by %d, t=%ld jiffies)\n",
                smp_processor_id(), (long)(jiffies - rsp->gp_start));
+        trigger_all_cpu_backtrace();
+
         force_quiescent_state(rsp, 0);  /* Kick them all. */
 }
 
@@ -479,12 +482,14 @@ static void print_cpu_stall(struct rcu_state *rsp)
 
         printk(KERN_ERR "INFO: RCU detected CPU %d stall (t=%lu jiffies)\n",
                         smp_processor_id(), jiffies - rsp->gp_start);
-        dump_stack();
+        trigger_all_cpu_backtrace();
+
         spin_lock_irqsave(&rnp->lock, flags);
         if ((long)(jiffies - rsp->jiffies_stall) >= 0)
                 rsp->jiffies_stall =
                         jiffies + RCU_SECONDS_TILL_STALL_RECHECK;
         spin_unlock_irqrestore(&rnp->lock, flags);
+
         set_need_resched();  /* kick ourselves to get things going. */
 }