linux-kernel - Re: [RFC][PATCH 12/12 v3] x86/nmi: Perform a safe NMI stack trace on all CPUs

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Date:	Tue, 4 Nov 2014 18:41:02 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Petr Mladek <pmladek@...e.cz>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC][PATCH 12/12 v3] x86/nmi: Perform a safe NMI stack trace
 on all CPUs

On Wed, 5 Nov 2014 00:05:12 +0100 (CET)
Jiri Kosina <jkosina@...e.cz> wrote:

> On Tue, 4 Nov 2014, Steven Rostedt wrote:
> 
> > From: "Steven Rostedt (Red Hat)" <rostedt@...dmis.org>
> > 
> > When trigger_all_cpu_backtrace() is called on x86, it will trigger an
> > NMI on each CPU and call show_regs(). But this can lead to a hard lock
> > up if the NMI comes in on another printk().
> > 
> > In order to avoid this, when the NMI triggers, it switches the printk
> > routine for that CPU to call a NMI safe printk function that records the
> > printk in a per_cpu seq_buf descriptor. After all NMIs have finished
> > recording its data, the trace_seqs are printed in a safe context.

Hmm, I need to update the change log to say seq_bufs instead of
trace_seqs.

> > 
> > Link: http://lkml.kernel.org/p/20140619213952.360076309@goodmis.org
> > 
> > Acked-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> 
> I've been running the whole machinery that used to trigger very quickly 
> the complete hardlock of the machine (*) for the whole evening/night, and 
> it's still running flawlessly.
> 
> Plus, as I said previously, I agree with the whole idea (given the 
> general nastiness of the problem and given the fact this simply has to be 
> fixed without pointless delays).
> 
> I.e FWIW
> 
> 	Tested-by: Jiri Kosina <jkosina@...e.cz>
> 	Acked-by: Jiri Kosina <jkosina@...e.cz>
> 
> for the whole series.

Thanks! I'll update the commits.

-- Steve

> 
> (*) heavy printk() workload (**) + sysrq-l in parallel
> (**) iptables logging every incoming packet + flood ping from another 
>      machine
> 
> Thanks,
> 

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