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[195.38.112.141]) by smtp.gmail.com with ESMTPSA id m18-20020aa7c492000000b0043a5bcf80a2sm1127022edq.60.2022.08.04.13.17.05 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 04 Aug 2022 13:17:06 -0700 (PDT) Sender: Ingo Molnar Date: Thu, 4 Aug 2022 22:17:04 +0200 From: Ingo Molnar To: Rik van Riel Cc: Dave Hansen , x86@kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, Thomas Gleixner , Dave Jones , Andy Lutomirski Subject: Re: [PATCH v2] x86,mm: print likely CPU at segfault time Message-ID: References: <20220804155450.08c5b87e@imladris.surriel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220804155450.08c5b87e@imladris.surriel.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Rik van Riel wrote: > 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 > CC: Dave Jones > --- > 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)); LGTM, applying this to tip:x86/mm unless someone objects. I've added the tidbit to the changelog that this only gets printed if show_unhandled_signals (/proc/sys/kernel/print-fatal-signals) is enabled - which is off by default. So your patch expands upon a default-off debug printout in essence - where utility maximization is OK. Thanks, Ingo