Re: [PATCH] x86: show remote CPU state upon fatal NMI

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 14/06/16 15:33, Jan Beulich wrote:
> Quite frequently the watchdog would hit an innocent CPU, e.g. one
> trying to acquire a spin lock a remote CPU holds for extended periods
> of time, or a random CPU in TSC calbration rendezvous. In such cases
> the register and stack dump for that CPU doesn't really help in the
> analysis of the problem.
>
> To keep things reasonable on large systems, only log CS:RIP by default.
> This can be overridden via a new extension to the "nmi=" command line
> option such that full register/stack state will get dumped.
>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
>
> --- a/docs/misc/xen-command-line.markdown
> +++ b/docs/misc/xen-command-line.markdown
> @@ -1155,7 +1155,7 @@ Use the MWAIT idle driver (with model sp
>  of the ACPI based one.
>  
>  ### nmi
> -> `= ignore | dom0 | fatal`
> +> `= ignore | dom0 | fatal [,show-all]`
>  
>  > Default: `fatal` for a debug build, or `dom0` for a non-debug build
>  
> @@ -1163,6 +1163,9 @@ Specify what Xen should do in the event
>  `ignore` discards the error; `dom0` causes Xen to report the error to
>  dom0, while 'fatal' causes Xen to print diagnostics and then hang.
>  
> +The `show-all` modifier forces all CPUs' full state to be dumped upon
> +fatal NMIs (normally a result of the watchdog kicking in).
> +
>  ### noapic
>  
>  Instruct Xen to ignore any IOAPICs that are present in the system, and
> --- a/xen/arch/x86/traps.c
> +++ b/xen/arch/x86/traps.c
> @@ -84,10 +84,11 @@
>   *  dom0:   The NMI is virtualised to DOM0.
>   *  ignore: The NMI error is cleared and ignored.
>   */
> +static char __read_mostly opt_nmi[16] =
>  #ifdef NDEBUG
> -static char __read_mostly opt_nmi[10] = "dom0";
> + "dom0";
>  #else
> -static char __read_mostly opt_nmi[10] = "fatal";
> + "fatal";
>  #endif
>  string_param("nmi", opt_nmi);
>  
> @@ -525,6 +526,35 @@ void vcpu_show_execution_state(struct vc
>      vcpu_unpause(v);
>  }
>  
> +static cpumask_t nmi_show_state_mask;
> +static bool_t opt_nmi_show_all;
> +
> +static int __init get_nmi_show_all(void)
> +{
> +    const char *s = strchr(opt_nmi, ',');
> +
> +    if ( s && !strcmp(s + 1, "show-all") )
> +        opt_nmi_show_all = 1;
> +
> +    return 0;
> +}
> +presmp_initcall(get_nmi_show_all);
> +
> +static int nmi_show_execution_state(const struct cpu_user_regs *regs, int cpu)
> +{
> +    if ( !cpumask_test_cpu(cpu, &nmi_show_state_mask) )
> +        return 0;
> +
> +    if ( opt_nmi_show_all )
> +        show_execution_state(regs);
> +    else
> +        printk(XENLOG_ERR "CPU%d @ %04x:%08lx (%pS)\n", cpu, regs->cs, regs->rip,
> +               guest_mode(regs) ? _p(regs->rip) : NULL);
> +    cpumask_clear_cpu(cpu, &nmi_show_state_mask);

I would clear the mask before printing state.  Given the nature of this
handler, it liable to contend sufficiently on the console lock to induce
the further watchdog timeout.

> +
> +    return 1;
> +}
> +
>  static const char *trapstr(unsigned int trapnr)
>  {
>      static const char * const strings[] = {
> @@ -570,6 +600,15 @@ void fatal_trap(const struct cpu_user_re
>              printk("Faulting linear address: %p\n", _p(cr2));
>              show_page_walk(cr2);
>          }
> +        else if ( trapnr == TRAP_nmi )
> +        {
> +            cpumask_andnot(&nmi_show_state_mask, &cpu_online_map,
> +                           cpumask_of(smp_processor_id()));
> +            set_nmi_callback(nmi_show_execution_state);
> +            smp_send_nmi_allbutself();

This would cause far less spinlock contention as

for_each_cpu( cpu, nmi_show_state_mask )
    smp_send_nmi(cpu);

I realise this involves introducing a new smp function, but it would
substantially reduce contention on the console lock.


I would recommend moving this clause into nmi_watchdog_tick(), so that
it doesn't get involved for non-watchdog NMIs.  IOCK/SERR NMIs won't
have anything interesting to print from here.  I would also recommend
disabling the watchdog before IPI'ing.

~Andrew

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