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Date:   Thu, 4 Aug 2022 15:54:50 -0400
From:   Rik van Riel <riel@surriel.com>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     x86@kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com,
        Thomas Gleixner <tglx@linutronix.de>, Dave Jones <dsj@fb.com>,
        Andy Lutomirski <luto@kernel.org>
Subject: [PATCH v2]  x86,mm: print likely CPU at segfault time
Message-ID: <20220804155450.08c5b87e@imladris.surriel.com>
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In a large enough fleet of computers, it is common to have a few bad CPUs.
Those can often be identified by seeing that some commonly run kernel code,
which runs fine everywhere else, keeps crashing on the same CPU core on one
particular bad system.

However, the failure modes in CPUs that have gone bad over the years are
often oddly specific, and the only bad behavior seen might be segfaults
in programs like bash, python, or various system daemons that run fine
everywhere else.

Add a printk() to show_signal_msg() to print the CPU, core, and socket
at segfault time. This is not perfect, since the task might get rescheduled
on another CPU between when the fault hit, and when the message is printed,
but in practice this has been good enough to help us identify several bad
CPU cores.

segfault[1349]: segfault at 0 ip 000000000040113a sp 00007ffc6d32e360 error 4 in segfault[401000+1000] on CPU 0 (core 0, socket 0)

Signed-off-by: Rik van Riel <riel@surriel.com>
CC: Dave Jones <dsj@fb.com>
---
 arch/x86/mm/fault.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index fad8faa29d04..a9b93a7816f9 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -769,6 +769,8 @@ show_signal_msg(struct pt_regs *regs, unsigned long error_code,
 		unsigned long address, struct task_struct *tsk)
 {
 	const char *loglvl = task_pid_nr(tsk) > 1 ? KERN_INFO : KERN_EMERG;
+	/* This is a racy snapshot, but it's better than nothing. */
+	int cpu = READ_ONCE(raw_smp_processor_id());
 
 	if (!unhandled_signal(tsk, SIGSEGV))
 		return;
@@ -782,6 +784,14 @@ show_signal_msg(struct pt_regs *regs, unsigned long error_code,
 
 	print_vma_addr(KERN_CONT " in ", regs->ip);
 
+	/*
+	 * Dump the likely CPU where the fatal segfault happened.
+	 * This can help identify faulty hardware.
+	 */
+	printk(KERN_CONT " on CPU %d (core %d, socket %d)", cpu,
+	       topology_core_id(cpu), topology_physical_package_id(cpu));
+
+
 	printk(KERN_CONT "\n");
 
 	show_opcodes(regs, loglvl);
-- 
2.37.1