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x86/nmi: Upgrade NMI backtrace stall checks & messages

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The commit to improve NMI stall debuggability:

  344da544f1 ("x86/nmi: Print reasons why backtrace NMIs are ignored")

... has shown value, but widespread use has also identified a few
opportunities for improvement.

The systems have (as usual) shown far more creativity than that commit's
author, demonstrating yet again that failing CPUs can do whatever they want.

In addition, the current message format is less friendly than one might
like to those attempting to use these messages to identify failing CPUs.

Therefore, separately flag CPUs that, during the full time that the
stack-backtrace request was waiting, were always in an NMI handler,
were never in an NMI handler, or exited one NMI handler.

Also, split the message identifying the CPU and the time since that CPU's
last NMI-related activity so that a single line identifies the CPU without
any other variable information, greatly reducing the processing overhead
required to identify repeat-offender CPUs.

Co-developed-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/ab4d70c8-c874-42dc-b206-643018922393@paulmck-laptop
This commit is contained in:
Paul E. McKenney 2024-03-07 17:32:15 -08:00 committed by Ingo Molnar
parent b6540de9b5
commit 3186b61812
1 changed files with 14 additions and 10 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

24
arch/x86/kernel/nmi.c
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@ -580,7 +580,7 @@ EXPORT_SYMBOL_GPL(asm_exc_nmi_kvm_vmx);
static char *nmi_check_stall_msg[] = { static char *nmi_check_stall_msg[] = {
/* */ /* */
/* +--------- nsp->idt_seq_snap & 0x1: CPU is in NMI handler. */ /* +--------- nmi_seq & 0x1: CPU is currently in NMI handler. */
/* | +------ cpu_is_offline(cpu) */ /* | +------ cpu_is_offline(cpu) */
/* | | +--- nsp->idt_calls_snap != atomic_long_read(&nsp->idt_calls): */ /* | | +--- nsp->idt_calls_snap != atomic_long_read(&nsp->idt_calls): */
/* | | | NMI handler has been invoked. */ /* | | | NMI handler has been invoked. */
@ -628,22 +628,26 @@ void nmi_backtrace_stall_check(const struct cpumask *btp)
nmi_seq = READ_ONCE(nsp->idt_nmi_seq); nmi_seq = READ_ONCE(nsp->idt_nmi_seq);
if (nsp->idt_nmi_seq_snap + 1 == nmi_seq && (nmi_seq & 0x1)) { if (nsp->idt_nmi_seq_snap + 1 == nmi_seq && (nmi_seq & 0x1)) {
msgp = "CPU entered NMI handler function, but has not exited"; msgp = "CPU entered NMI handler function, but has not exited";
} else if ((nsp->idt_nmi_seq_snap & 0x1) != (nmi_seq & 0x1)) { } else if (nsp->idt_nmi_seq_snap == nmi_seq ||
msgp = "CPU is handling NMIs"; nsp->idt_nmi_seq_snap + 1 == nmi_seq) {
} else { idx = ((nmi_seq & 0x1) << 2) |
idx = ((nsp->idt_seq_snap & 0x1) << 2) |
(cpu_is_offline(cpu) << 1) | (cpu_is_offline(cpu) << 1) |
(nsp->idt_calls_snap != atomic_long_read(&nsp->idt_calls)); (nsp->idt_calls_snap != atomic_long_read(&nsp->idt_calls));
msgp = nmi_check_stall_msg[idx]; msgp = nmi_check_stall_msg[idx];
if (nsp->idt_ignored_snap != READ_ONCE(nsp->idt_ignored) && (idx & 0x1)) if (nsp->idt_ignored_snap != READ_ONCE(nsp->idt_ignored) && (idx & 0x1))
modp = ", but OK because ignore_nmis was set"; modp = ", but OK because ignore_nmis was set";
if (nmi_seq & 0x1) if (nsp->idt_nmi_seq_snap + 1 == nmi_seq)
msghp = " (CPU currently in NMI handler function)";
else if (nsp->idt_nmi_seq_snap + 1 == nmi_seq)
msghp = " (CPU exited one NMI handler function)"; msghp = " (CPU exited one NMI handler function)";
else if (nmi_seq & 0x1)
msghp = " (CPU currently in NMI handler function)";
else
msghp = " (CPU was never in an NMI handler function)";
} else {
msgp = "CPU is handling NMIs";
} }
pr_alert("%s: CPU %d: %s%s%s, last activity: %lu jiffies ago.\n", pr_alert("%s: CPU %d: %s%s%s\n", __func__, cpu, msgp, modp, msghp);
__func__, cpu, msgp, modp, msghp, j - READ_ONCE(nsp->recv_jiffies)); pr_alert("%s: last activity: %lu jiffies ago.\n",
__func__, j - READ_ONCE(nsp->recv_jiffies));
} }
} }