Re: [RFC][PATCH 3/3] x86/nmi: Perform a safe NMI stack trace on all CPUs

Paul E. McKenney
2014-06-23T16:12:26Z

On Thu, Jun 19, 2014 at 05:33:32PM -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>
>
> When trigger_all_cpu_backtrace() is called on x86, it will trigger an
> NMI on each CPU and call show_regs(). But this can lead to a hard lock
> up if the NMI comes in on another printk().
>
> In order to avoid this, when the NMI triggers, it switches the printk
> routine for that CPU to call a NMI safe printk function that records the
> printk in a per_cpu trace_seq descriptor. After all NMIs have finished
> recording its data, the trace_seqs are printed in a safe context.
>
> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

I do like the idea of safe printk from NMI, so given the changes later
in this thread:

Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

> ---
> arch/x86/kernel/apic/hw_nmi.c | 66 ++++++++++++++++++++++++++++++++++++++++---
> 1 file changed, 62 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kernel/apic/hw_nmi.c b/arch/x86/kernel/apic/hw_nmi.c
> index c3fcb5de5083..6731604bb1cd 100644
> --- a/arch/x86/kernel/apic/hw_nmi.c
> +++ b/arch/x86/kernel/apic/hw_nmi.c
> @@ -18,6 +18,7 @@
> #include <linux/nmi.h>
> #include <linux/module.h>
> #include <linux/delay.h>
> +#include <linux/trace_seq.h>
>
> #ifdef CONFIG_HARDLOCKUP_DETECTOR
> u64 hw_nmi_get_sample_period(int watchdog_thresh)
> @@ -30,11 +31,30 @@ u64 hw_nmi_get_sample_period(int watchdog_thresh)
> /* For reliability, we're prepared to waste bits here. */
> static DECLARE_BITMAP(backtrace_mask, NR_CPUS) __read_mostly;
>
> +/* Safe printing in NMI context */
> +static DEFINE_PER_CPU(struct trace_seq, nmi_print_seq);
> +
> /* "in progress" flag of arch_trigger_all_cpu_backtrace */
> static unsigned long backtrace_flag;
>
> +static void print_seq_line(struct trace_seq *s, int last, int pos)
> +{
> + const char *buf = s->buffer + last;
> +
> + /* Chop off the saved log level and update the length */
> + if (printk_get_level(buf)) {
> + buf += 2;
> + last += 2;
> + }
> +
> + pr_emerg("%.*s", (pos - last) + 1, buf);
> +}
> +
> void arch_trigger_all_cpu_backtrace(void)
> {
> + struct trace_seq *s;
> + int len;
> + int cpu;
> int i;
>
> if (test_and_set_bit(0, &backtrace_flag))
> @@ -44,6 +64,11 @@ void arch_trigger_all_cpu_backtrace(void)
> */
> return;
>
> + for_each_possible_cpu(i) {
> + s = &per_cpu(nmi_print_seq, i);
> + trace_seq_init(s);
> + }
> +
> cpumask_copy(to_cpumask(backtrace_mask), cpu_online_mask);
>
> printk(KERN_INFO "sending NMI to all CPUs:\n");
> @@ -56,8 +81,39 @@ void arch_trigger_all_cpu_backtrace(void)
> mdelay(1);
> }
>
> + for_each_possible_cpu(cpu) {
> + int last_i = 0;
> +
> + s = &per_cpu(nmi_print_seq, cpu);
> + len = s->len;
> + if (!len)
> + continue;
> +
> + /* Print line by line. */
> + for (i = 0; i < len; i++) {
> + if (s->buffer[i] == '\n') {
> + print_seq_line(s, last_i, i);
> + last_i = i + 1;
> + }
> + }
> + if (last_i < i - 1) {
> + print_seq_line(s, last_i, i);
> + pr_cont("\n");
> + }
> + }
> +
> clear_bit(0, &backtrace_flag);
> smp_mb__after_atomic();
> + return true;
> +}
> +
> +static int nmi_vprintk(const char *fmt, va_list args)
> +{
> + struct trace_seq *s = this_cpu_ptr(&nmi_print_seq);
> + int len = s->len;
> +
> + trace_seq_vprintf(s, fmt, args);
> + return s->len - len;
> }
>
> static int
> @@ -68,12 +124,14 @@ arch_trigger_all_cpu_backtrace_handler(unsigned int cmd, struct pt_regs *regs)
> cpu = smp_processor_id();
>
> if (cpumask_test_cpu(cpu, to_cpumask(backtrace_mask))) {
> - static arch_spinlock_t lock = __ARCH_SPIN_LOCK_UNLOCKED;
> + printk_func_t printk_func_save = this_cpu_read(printk_func);
>
> - arch_spin_lock(&lock);
> - printk(KERN_WARNING "NMI backtrace for cpu %d\n", cpu);
> + /* Replace printk to write into the NMI seq */
> + this_cpu_write(printk_func, nmi_vprintk);
> + printk("NMI backtrace for cpu %d\n", cpu);
> show_regs(regs);
> - arch_spin_unlock(&lock);
> + this_cpu_write(printk_func, printk_func_save);
> +
> cpumask_clear_cpu(cpu, to_cpumask(backtrace_mask));
> return NMI_HANDLED;
> }
> --
> 2.0.0.rc2
>
>
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