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Date: Mon, 19 Jul 2021 15:00:41 -0400 From: Rik van Riel <riel@...riel.com> To: linux-kernel@...r.kernel.org Cc: Dave Hansen <dave.hansen@...ux.intel.com>, Andy Lutomirski <luto@...nel.org>, kernel-team@...com, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, x86@...nel.org Subject: [PATCH] x86,mm: print likely CPU at segfault time >From 14d31a44a5186c94399dc9518ba80adf64c99772 Mon Sep 17 00:00:00 2001 From: Rik van Riel <riel@...ters.home.surriel.com> Date: Mon, 19 Jul 2021 14:49:17 -0400 Subject: [PATCH] x86,mm: print likely CPU at segfault time 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 (that runs fine everywhere else) keeps crashing on the same CPU core on a particular bad system. One of the failure modes observed is that either the instruction pointer, or some register used to specify the address of data that needs to be fetched gets corrupted, resulting in something like a kernel page fault, null pointer dereference, NX violation, or similar. Those kernel failures are often preceded by similar looking userspace failures. It would be useful to know if those are also happening on the same CPU cores, to get a little more confirmation that it is indeed a hardware issue. Adding a printk to show_signal_msg() achieves that purpose. It isn't perfect since the task might get rescheduled on another CPU between when the fault hit and when the message is printed, but it should be good enough to show correlation between userspace and kernel errors when dealing with a bad CPU. $ ./segfault Segmentation fault (core dumped) $ dmesg | grep segfault segfault[1349]: segfault at 0 ip 000000000040113a sp 00007ffc6d32e360 error 4 in segfault[401000+1000] on CPU 0 Signed-off-by: Rik van Riel <riel@...riel.com> --- arch/x86/mm/fault.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c index b2eefdefc108..dd6c89c23a3a 100644 --- a/arch/x86/mm/fault.c +++ b/arch/x86/mm/fault.c @@ -777,6 +777,8 @@ show_signal_msg(struct pt_regs *regs, unsigned long error_code, print_vma_addr(KERN_CONT " in ", regs->ip); + printk(KERN_CONT " on CPU %d", raw_smp_processor_id()); + printk(KERN_CONT "\n"); show_opcodes(regs, loglvl); -- 2.24.1
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