Commit c1dc0b9c authored by Ingo Molnar's avatar Ingo Molnar
Browse files

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

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 <>
Cc: Peter Zijlstra <>
Cc: Andrew Morton <>
Cc: Linus Torvalds <>
LKML-Reference: <new-submission>
Signed-off-by: default avatarIngo Molnar <>
parent ed680c4a
......@@ -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 ... ");
......@@ -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 */
printk(KERN_WARNING "NMI backtrace for cpu %d\n", cpu);
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");
/* 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))
......@@ -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());
static struct sysrq_key_op sysrq_showallcpus_op = {
......@@ -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));
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);
spin_lock_irqsave(&rnp->lock, flags);
if ((long)(jiffies - rsp->jiffies_stall) >= 0)
rsp->jiffies_stall =
spin_unlock_irqrestore(&rnp->lock, flags);
set_need_resched(); /* kick ourselves to get things going. */
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