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x86/nmi: Print reasons why backtrace NMIs are ignored

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Instrument nmi_trigger_cpumask_backtrace() to dump out diagnostics based
on evidence accumulated by exc_nmi().  These diagnostics are dumped for
CPUs that ignored an NMI backtrace request for more than 10 seconds.

[ paulmck: Apply Ingo Molnar feedback. ]

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: <x86@kernel.org>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Paul E. McKenney 2022-12-16 16:52:01 -08:00
parent 1a3ea611fc
commit 344da544f1
3 changed files with 83 additions and 0 deletions
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73
arch/x86/kernel/nmi.c
View File

@ -570,6 +570,79 @@ DEFINE_IDTENTRY_RAW(exc_nmi_noist)
EXPORT_SYMBOL_GPL(asm_exc_nmi_noist);
#endif
#ifdef CONFIG_NMI_CHECK_CPU
static char *nmi_check_stall_msg[] = {
/* */
/* +--------- nsp->idt_seq_snap & 0x1: CPU is in NMI handler. */
/* | +------ cpu_is_offline(cpu) */
/* | | +--- nsp->idt_calls_snap != atomic_long_read(&nsp->idt_calls): */
/* | | | NMI handler has been invoked. */
/* | | | */
/* V V V */
/* 0 0 0 */ "NMIs are not reaching exc_nmi() handler",
/* 0 0 1 */ "exc_nmi() handler is ignoring NMIs",
/* 0 1 0 */ "CPU is offline and NMIs are not reaching exc_nmi() handler",
/* 0 1 1 */ "CPU is offline and exc_nmi() handler is legitimately ignoring NMIs",
/* 1 0 0 */ "CPU is in exc_nmi() handler and no further NMIs are reaching handler",
/* 1 0 1 */ "CPU is in exc_nmi() handler which is legitimately ignoring NMIs",
/* 1 1 0 */ "CPU is offline in exc_nmi() handler and no more NMIs are reaching exc_nmi() handler",
/* 1 1 1 */ "CPU is offline in exc_nmi() handler which is legitimately ignoring NMIs",
};
void nmi_backtrace_stall_snap(const struct cpumask *btp)
{
int cpu;
struct nmi_stats *nsp;
for_each_cpu(cpu, btp) {
nsp = per_cpu_ptr(&nmi_stats, cpu);
nsp->idt_seq_snap = READ_ONCE(nsp->idt_seq);
nsp->idt_nmi_seq_snap = READ_ONCE(nsp->idt_nmi_seq);
nsp->idt_ignored_snap = READ_ONCE(nsp->idt_ignored);
nsp->idt_calls_snap = atomic_long_read(&nsp->idt_calls);
}
}
void nmi_backtrace_stall_check(const struct cpumask *btp)
{
int cpu;
int idx;
unsigned long nmi_seq;
unsigned long j = jiffies;
char *modp;
char *msgp;
char *msghp;
struct nmi_stats *nsp;
for_each_cpu(cpu, btp) {
nsp = per_cpu_ptr(&nmi_stats, cpu);
modp = "";
msghp = "";
nmi_seq = READ_ONCE(nsp->idt_nmi_seq);
if (nsp->idt_nmi_seq_snap + 1 == nmi_seq && (nmi_seq & 0x1)) {
msgp = "CPU entered NMI handler function, but has not exited";
} else if ((nsp->idt_nmi_seq_snap & 0x1) != (nmi_seq & 0x1)) {
msgp = "CPU is handling NMIs";
} else {
idx = ((nsp->idt_seq_snap & 0x1) << 2) |
(cpu_is_offline(cpu) << 1) |
(nsp->idt_calls_snap != atomic_long_read(&nsp->idt_calls));
msgp = nmi_check_stall_msg[idx];
if (nsp->idt_ignored_snap != READ_ONCE(nsp->idt_ignored) && (idx & 0x1))
modp = ", but OK because ignore_nmis was set";
if (nmi_seq & ~0x1)
msghp = " (CPU currently in NMI handler function)";
else if (nsp->idt_nmi_seq_snap + 1 == nmi_seq)
msghp = " (CPU exited one NMI handler function)";
}
pr_alert("%s: CPU %d: %s%s%s, last activity: %lu jiffies ago.\n",
__func__, cpu, msgp, modp, msghp, j - READ_ONCE(nsp->recv_jiffies));
}
}
#endif
void stop_nmi(void)
{
ignore_nmis++;

8
include/linux/nmi.h
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@ -214,4 +214,12 @@ int proc_watchdog_cpumask(struct ctl_table *, int, void *, size_t *, loff_t *);
#include <asm/nmi.h>
#endif
#ifdef CONFIG_NMI_CHECK_CPU
void nmi_backtrace_stall_snap(const struct cpumask *btp);
void nmi_backtrace_stall_check(const struct cpumask *btp);
#else
static inline void nmi_backtrace_stall_snap(const struct cpumask *btp) {}
static inline void nmi_backtrace_stall_check(const struct cpumask *btp) {}
#endif
#endif

2
lib/nmi_backtrace.c
View File

@ -64,6 +64,7 @@ void nmi_trigger_cpumask_backtrace(const cpumask_t *mask,
if (!cpumask_empty(to_cpumask(backtrace_mask))) {
pr_info("Sending NMI from CPU %d to CPUs %*pbl:\n",
this_cpu, nr_cpumask_bits, to_cpumask(backtrace_mask));
nmi_backtrace_stall_snap(to_cpumask(backtrace_mask));
raise(to_cpumask(backtrace_mask));
}
@ -74,6 +75,7 @@ void nmi_trigger_cpumask_backtrace(const cpumask_t *mask,
mdelay(1);
touch_softlockup_watchdog();
}
nmi_backtrace_stall_check(to_cpumask(backtrace_mask));
/*
* Force flush any remote buffers that might be stuck in IRQ context