linux-kernel - Re: + panic-avoid-the-extra-noise-dmesg.patch added to -mm tree

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Date:   Tue, 11 Dec 2018 17:00:43 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Feng Tang <feng.tang@...el.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>, akpm@...ux-foundation.org,
        bp@...e.de, keescook@...omium.org, mm-commits@...r.kernel.org,
        sergey.senozhatsky@...il.com, stable@...r.kernel.org,
        tglx@...utronix.de, Steven Rostedt <rostedt@...dmis.org>,
        Sasha Levin <sashal@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>, linux-kernel@...r.kernel.org
Subject: Re: + panic-avoid-the-extra-noise-dmesg.patch added to -mm tree

Hi,

On (12/10/18 17:45), Feng Tang wrote:
> Yes, this is very valid concern. And after Petr and you raised it, I did
> some experiments with 3 x86 platforms at my hand, one Apollolake IOT device
> with serial console, one IvyBridge laptop and one Kabylake NUC, the magic key
> all works well before panic, and fails after panic. But I did remember the
> PageUp/PageDown key worked on some laptop years ago. And you actually raised a
> good question: what do we expect for the post-panic kernel?

Yeah.
It used to be case that people expected some things to work after panic.

> For the v4 patch, my thought is, for experienced developers to make
> sysrq/panic_blink work, it's easy to add "panic_keep_irq_on" to kernel cmdline,
> or runtime change it by
>  "echo Y > /sys/module/kernel/parameters/panic_keep_irq_on"
> while for normal user, they can by default see the clean panic call stack
> either on a screen or a serial console.

Before we move on, just a quick question, since I wasn't Cc-ed to v1
and v2 of this patch - did you have a chance to ask x86 people if they
can help in any way? Asking to make sure that we are not fixing a _maybe_
x86-specific problem in arch-independent/common code.


/* offtopic */

LOL, wish this was a "dumb-and-ugly-solutions" contest; I'm pretty sure
I'd take the first prize with this one:

---
diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
index 04adc8d60aed..40f643bb7fdc 100644
--- a/arch/x86/kernel/smp.c
+++ b/arch/x86/kernel/smp.c
@@ -181,6 +181,16 @@ asmlinkage __visible void smp_reboot_interrupt(void)
 	irq_exit();
 }
 
+static void native_smp_suppress_reschedule(int cpu)
+{
+}
+
+static void native_smp_to_up(void)
+{
+	WARN_ON_ONCE(num_online_cpus() > 1);
+	smp_ops.smp_send_reschedule = native_smp_suppress_reschedule;
+}
+
 static void native_stop_other_cpus(int wait)
 {
 	unsigned long flags;
@@ -250,6 +260,7 @@ static void native_stop_other_cpus(int wait)
 	local_irq_save(flags);
 	disable_local_APIC();
 	mcheck_cpu_clear(this_cpu_ptr(&cpu_info));
+	native_smp_to_up();
 	local_irq_restore(flags);
 }
---

If the system is not SMP anymore (hlt non-panic CPUs) - rewrite
some smp_ops pointers to NOOP stubs to suppress some of those
warnings.

I know it's utterly awful.

	-ss