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SubjectRe: [PATCH] sysrq show-all-cpus
On Mon, 10 Mar 2008 13:32:42 -0400
Rik van Riel <riel@redhat.com> wrote:

> 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@redhat.com>
>
> 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 = "show-all-cpus(L)",
> + .action_msg = "Show backtrace of all active CPUs",
> + .enable_mask = SYSRQ_ENABLE_DUMP,
> +};
> +#endif

So the CPU which took the interrupt is always the first to display - that's
nice.

Even if it's idle. I guess that's OK.

How come the sysrq-handling CPU uses show_regs() and the others use
show_stack()? Do we not set up the get_irq_regs() data for IPIs?

> 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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