linux-kernel - Re: [PATCH RFC] panic: Avoid extra noisy messages due to stopped cpus

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Date:   Thu, 11 Oct 2018 11:35:00 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Feng Tang <feng.tang@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        H Peter Anvin <hpa@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] panic: Avoid extra noisy messages due to stopped cpus

On Thu, Oct 11, 2018 at 03:17:48PM +0800, Feng Tang wrote:
> Sometimes when debugging kernel panic, we saw many extra noisy error
> messages after the expected end:
> 
> [   35.743249] ---[ end Kernel panic - not syncing: Fatal exception
> [   35.749975] ------------[ cut here ]------------
> 
> These messages may overflow the sceen (framebuffer) and make debugging
> much difficulter.

*blink* you actually using the framebuffer for debugging ?! Why the heck
are you doing that?

> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index c93fcfd..b703862 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -520,6 +520,7 @@ void stop_this_cpu(void *dummy)
>  	 * Remove this CPU:
>  	 */
>  	set_cpu_online(smp_processor_id(), false);
> +	set_cpu_active(smp_processor_id(), false);
>  	disable_local_APIC();
>  	mcheck_cpu_clear(this_cpu_ptr(&cpu_info));
>  

WTH is stop_this_cpu() and how do we even get here with active still
set?

> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 7fc4a37..cf41b7b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9034,7 +9034,7 @@ static inline int find_new_ilb(void)
>  {
>  	int ilb = cpumask_first(nohz.idle_cpus_mask);
>  
> -	if (ilb < nr_cpu_ids && idle_cpu(ilb))
> +	if (ilb < nr_cpu_ids && idle_cpu(ilb) && cpu_online(ilb))
>  		return ilb;
>  
>  	return nr_cpu_ids;


Similar, this is the result of taking the CPU away without going through
the normal path. You're doing something dodgy.