[PATCH] sysrq show-all-cpus

[Rik van Riel]

SysRQ-P is not always useful on SMP systems, since it usually ends up showing the
backtrace of a CPU that is doing just fine, instead of the backtrace of the CPU
that is having problems.

This patch adds SysRQ show-all-cpus(L), which shows the backtrace of every active
CPU in the system.  It skips idle CPUs because some SMP systems are just too large
and we already know what the backtrace of the idle task looks like.
 
Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>

diff -up linux-2.6.25-rc3-mm1/Documentation/sysrq.txt.multicpu linux-2.6.25-rc3-mm1/Documentation/sysrq.txt
--- linux-2.6.25-rc3-mm1/Documentation/sysrq.txt.multicpu	2008-03-10 13:49:18.000000000 -0400
+++ linux-2.6.25-rc3-mm1/Documentation/sysrq.txt	2008-03-10 13:51:09.000000000 -0400
@@ -85,6 +85,8 @@ On all -  write a character to /proc/sys
 'k'     - Secure Access Key (SAK) Kills all programs on the current virtual
           console. NOTE: See important comments below in SAK section.
 
+'l'     - Shows a stack backtrace for all active CPUs.
+
 'm'     - Will dump current memory info to your console.
 
 'n'	- Used to make RT tasks nice-able
diff -up linux-2.6.25-rc3-mm1/drivers/char/sysrq.c.multicpu linux-2.6.25-rc3-mm1/drivers/char/sysrq.c
--- linux-2.6.25-rc3-mm1/drivers/char/sysrq.c.multicpu	2008-03-09 20:22:17.000000000 -0400
+++ linux-2.6.25-rc3-mm1/drivers/char/sysrq.c	2008-03-10 13:03:00.000000000 -0400
@@ -196,6 +196,44 @@ static struct sysrq_key_op sysrq_showloc
 #define sysrq_showlocks_op (*(struct sysrq_key_op *)0)
 #endif
 
+#ifdef CONFIG_SMP
+static DEFINE_SPINLOCK(show_lock);
+static void showacpu(void *dummy)
+{
+	unsigned long flags;
+
+	/* Idle CPUs have no interesting backtrace. */
+	if (idle_cpu(smp_processor_id()))
+		return;
+
+	spin_lock_irqsave(&show_lock, flags);
+	printk("CPU%d:\n", smp_processor_id());
+	show_stack(NULL, NULL);
+	spin_unlock_irqrestore(&show_lock, flags);
+}
+static void sysrq_showregs_othercpus(struct work_struct *dummy)
+{
+	smp_call_function(showacpu, NULL, 0, 0);
+}
+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("CPU%d:\n", smp_processor_id());
+		show_regs(regs);
+	}
+	schedule_work(&sysrq_showallcpus);
+}
+static struct sysrq_key_op sysrq_showallcpus_op = {
+	.handler	= sysrq_handle_showallcpus,
+	.help_msg	= "aLlcpus",
+	.action_msg	= "Show backtrace of all active CPUs",
+	.enable_mask	= SYSRQ_ENABLE_DUMP,
+};
+#endif
+
 static void sysrq_handle_showregs(int key, struct tty_struct *tty)
 {
 	struct pt_regs *regs = get_irq_regs();
@@ -340,7 +378,11 @@ static struct sysrq_key_op *sysrq_key_ta
 	&sysrq_kill_op,			/* i */
 	NULL,				/* j */
 	&sysrq_SAK_op,			/* k */
+#ifdef CONFIG_SMP
+	&sysrq_showallcpus_op,		/* l */
+#else
 	NULL,				/* l */
+#endif
 	&sysrq_showmem_op,		/* m */
 	&sysrq_unrt_op,			/* n */
 	/* o: This will often be registered as 'Off' at init time */
--
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