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Subject[patch] softlockup: improve debug output
Subject: softlockup: improve debug output
From: Ingo Molnar <mingo@elte.hu>

improve the debuggability of kernel lockups by enhancing the debug
output of the softlockup detector: print the task that causes the lockup
and try to print a more intelligent backtrace.

the old format was:

BUG: soft lockup detected on CPU#1!
[<c0105e4a>] show_trace_log_lvl+0x19/0x2e
[<c0105f43>] show_trace+0x12/0x14
[<c0105f59>] dump_stack+0x14/0x16
[<c015f6bc>] softlockup_tick+0xbe/0xd0
[<c013457d>] run_local_timers+0x12/0x14
[<c01346b8>] update_process_times+0x3e/0x63
[<c0145fb8>] tick_sched_timer+0x7c/0xc0
[<c0140a75>] hrtimer_interrupt+0x135/0x1ba
[<c011bde7>] smp_apic_timer_interrupt+0x6e/0x80
[<c0105aa3>] apic_timer_interrupt+0x33/0x38
[<c0104f8a>] syscall_call+0x7/0xb
=======================
the new format is:

BUG: soft lockup detected on CPU#1! [prctl:2363]

Pid: 2363, comm: prctl
EIP: 0060:[<c013915f>] CPU: 1
EIP is at sys_prctl+0x24/0x18c
EFLAGS: 00000213 Not tainted (2.6.22-cfs-v20 #26)
EAX: 00000001 EBX: 000003e7 ECX: 00000001 EDX: f6df0000
ESI: 000003e7 EDI: 000003e7 EBP: f6df0fb0 DS: 007b ES: 007b FS: 00d8
CR0: 8005003b CR2: 4d8c3340 CR3: 3731d000 CR4: 000006d0
[<c0105e4a>] show_trace_log_lvl+0x19/0x2e
[<c0105f43>] show_trace+0x12/0x14
[<c01040be>] show_regs+0x1ab/0x1b3
[<c015f807>] softlockup_tick+0xef/0x108
[<c013457d>] run_local_timers+0x12/0x14
[<c01346b8>] update_process_times+0x3e/0x63
[<c0145fcc>] tick_sched_timer+0x7c/0xc0
[<c0140a89>] hrtimer_interrupt+0x135/0x1ba
[<c011bde7>] smp_apic_timer_interrupt+0x6e/0x80
[<c0105aa3>] apic_timer_interrupt+0x33/0x38
[<c0104f8a>] syscall_call+0x7/0xb
=======================
note that in the old format we only knew that some system call locked
up, we didnt know _which_. With the new format we know that it's at a
specific place in sys_prctl(). [which was where i created an artificial
kernel lockup to test the new format.]

this is also useful if the lockup happens in user-space - the user-space
EIP (and other registers) will be printed too. (such a lockup would
either suggest that the task was running at SCHED_FIFO:99 and looping
for more than 10 seconds, or that the softlockup detector has a
false-positive.)

the task name is printed too first, just in case we dont manage to print
a useful backtrace.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
kernel/softlockup.c | 23 ++++++++++++++++-------
1 file changed, 16 insertions(+), 7 deletions(-)
Index: linux/kernel/softlockup.c
===================================================================
--- linux.orig/kernel/softlockup.c
+++ linux/kernel/softlockup.c
@@ -14,6 +14,8 @@
#include <linux/notifier.h>
#include <linux/module.h>

+#include <asm/irq_regs.h>
+
static DEFINE_SPINLOCK(print_lock);

static DEFINE_PER_CPU(unsigned long, touch_timestamp);
@@ -69,6 +71,7 @@ void softlockup_tick(void)
int this_cpu = smp_processor_id();
unsigned long touch_timestamp = per_cpu(touch_timestamp, this_cpu);
unsigned long print_timestamp;
+ struct pt_regs *regs;
unsigned long now;

if (touch_timestamp == 0) {
@@ -98,15 +101,21 @@ void softlockup_tick(void)
wake_up_process(per_cpu(watchdog_task, this_cpu));

/* Warn about unreasonable 10+ seconds delays: */
- if (now > (touch_timestamp + 10)) {
- per_cpu(print_timestamp, this_cpu) = touch_timestamp;
+ if (now <= (touch_timestamp + 10))
+ return;
+
+ regs = get_irq_regs();

- spin_lock(&print_lock);
- printk(KERN_ERR "BUG: soft lockup detected on CPU#%d!\n",
- this_cpu);
+ per_cpu(print_timestamp, this_cpu) = touch_timestamp;
+
+ spin_lock(&print_lock);
+ printk(KERN_ERR "BUG: soft lockup detected on CPU#%d! [%s:%d]\n",
+ this_cpu, current->comm, current->pid);
+ if (regs)
+ show_regs(regs);
+ else
dump_stack();
- spin_unlock(&print_lock);
- }
+ spin_unlock(&print_lock);
}

/*
-
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