On Thu 2014-06-26 17:49:06, Steven Rostedt wrote:
> From: "Steven Rostedt (Red Hat)" <[email protected]>
>
> 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 seq_buf descriptor. After all NMIs have finished
> recording its data, the trace_seqs are printed in a safe context.
>
> Link: http://lkml.kernel.org/p/[email protected]
>
> Acked-by: Paul E. McKenney <[email protected]>
> Signed-off-by: Steven Rostedt <[email protected]>
It seems to work very well.
Tested-by: Petr Mladek <[email protected]>
See one comment below.
> ---
> V2 - updated to use seq_buf instead of trace_seq.
> - Keep original printk loglevel, do not strip it, but use it.
> (suggested by Petr Ml?dek)
> - Updated comments
> - Removed return of true (noticed by Petr Ml?dek)
> ---
> arch/x86/kernel/apic/hw_nmi.c | 84 +++++++++++++++++++++++++++++++++++++++++--
> 1 file changed, 81 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kernel/apic/hw_nmi.c b/arch/x86/kernel/apic/hw_nmi.c
> index c3fcb5de5083..91ddba672839 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/seq_buf.h>
>
> #ifdef CONFIG_HARDLOCKUP_DETECTOR
> u64 hw_nmi_get_sample_period(int watchdog_thresh)
> @@ -30,11 +31,31 @@ 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;
>
> +#define NMI_BUF_SIZE 4096
> +
> +struct nmi_seq_buf {
> + unsigned char buffer[NMI_BUF_SIZE];
> + struct seq_buf seq;
> +};
> +
> +/* Safe printing in NMI context */
> +static DEFINE_PER_CPU(struct nmi_seq_buf, nmi_print_seq);
> +
> /* "in progress" flag of arch_trigger_all_cpu_backtrace */
> static unsigned long backtrace_flag;
>
> +static void print_seq_line(struct nmi_seq_buf *s, int last, int pos)
> +{
> + const char *buf = s->buffer + last;
> +
> + printk("%.*s", (pos - last) + 1, buf);
> +}
> +
> void arch_trigger_all_cpu_backtrace(void)
> {
> + struct nmi_seq_buf *s;
> + int len;
> + int cpu;
> int i;
>
> if (test_and_set_bit(0, &backtrace_flag))
> @@ -44,6 +65,15 @@ void arch_trigger_all_cpu_backtrace(void)
> */
> return;
>
> + /*
> + * Set up per_cpu seq_buf buffers that the NMIs running on the other
> + * CPUs will write to.
> + */
> + for_each_possible_cpu(cpu) {
> + s = &per_cpu(nmi_print_seq, cpu);
> + seq_buf_init(&s->seq, s->buffer, NMI_BUF_SIZE);
> + }
> +
> cpumask_copy(to_cpumask(backtrace_mask), cpu_online_mask);
>
> printk(KERN_INFO "sending NMI to all CPUs:\n");
> @@ -56,10 +86,56 @@ void arch_trigger_all_cpu_backtrace(void)
> mdelay(1);
> }
>
> + /*
> + * Now that all the NMIs have triggered, we can dump out their
> + * back traces safely to the console.
> + */
> + for_each_possible_cpu(cpu) {
> + int last_i = 0;
> +
> + s = &per_cpu(nmi_print_seq, cpu);
> + len = s->seq.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();
> }
>
> +/*
> + * It is not safe to call printk() directly from NMI handlers.
> + * It may be fine if the NMI detected a lock up and we have no choice
> + * but to do so, but doing a NMI on all other CPUs to get a back trace
> + * can be done with a sysrq-l. We don't want that to lock up, which
> + * can happen if the NMI interrupts a printk in progress.
> + *
> + * Instead, we redirect the vprintk() to this nmi_vprintk() that writes
> + * the content into a per cpu seq_buf buffer. Then when the NMIs are
> + * all done, we can safely dump the contents of the seq_buf to a printk()
> + * from a non NMI context.
> + */
I would move this comment above #define NMI_BUF_SIZE
It should be on top because it helps to understand many other tricks that are
used above.
Regards,
Petr
> +static int nmi_vprintk(const char *fmt, va_list args)
> +{
> + struct nmi_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
> + unsigned int len = s->seq.len;
> +
> + seq_buf_vprintf(&s->seq, fmt, args);
> + return s->seq.len - len;
> +}
> +
> static int
> arch_trigger_all_cpu_backtrace_handler(unsigned int cmd, struct pt_regs *regs)
> {
> @@ -68,12 +144,14 @@ 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);
> + /* Replace printk to write into the NMI seq */
> + this_cpu_write(printk_func, nmi_vprintk);
> printk(KERN_WARNING "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
>
>
On Fri, 27 Jun 2014 16:32:46 +0200
Petr Mládek <[email protected]> wrote:
> > +/*
> > + * It is not safe to call printk() directly from NMI handlers.
> > + * It may be fine if the NMI detected a lock up and we have no choice
> > + * but to do so, but doing a NMI on all other CPUs to get a back trace
> > + * can be done with a sysrq-l. We don't want that to lock up, which
> > + * can happen if the NMI interrupts a printk in progress.
> > + *
> > + * Instead, we redirect the vprintk() to this nmi_vprintk() that writes
> > + * the content into a per cpu seq_buf buffer. Then when the NMIs are
> > + * all done, we can safely dump the contents of the seq_buf to a printk()
> > + * from a non NMI context.
> > + */
>
> I would move this comment above #define NMI_BUF_SIZE
> It should be on top because it helps to understand many other tricks that are
> used above.
>
I have no problem with moving it. This started out as a simple comment
and then I just got carried away ;-)
-- Steve