linux-kernel - Re: [RFC][PATCH 0/3] x86/nmi: Print all cpu stacks from NMI safely

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Message-ID: <alpine.LNX.2.00.1406200100520.15014@pobox.suse.cz>
Date:	Fri, 20 Jun 2014 01:03:28 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>, Jan Kara <jack@...e.cz>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Dave Anderson <anderson@...hat.com>,
	Petr Mladek <pmladek@...e.cz>
Subject: Re: [RFC][PATCH 0/3] x86/nmi: Print all cpu stacks from NMI safely

On Thu, 19 Jun 2014, Steven Rostedt wrote:

> > The idea basically is to *switch* what arch_trigger_all_cpu_backtrace() 
> > and arch_trigger_all_cpu_backtrace_handler() are doing; i.e. use the NMI 
> > as a way to stop all the CPUs (one by one), and let the CPU that is 
> > sending the NMIs around to actually walk and dump the stacks of the CPUs 
> > receiving the NMI IPI.
> 
> And this is cleaner? Stopping a CPU via NMI and then what happens if
> something else goes wrong and that CPU never starts back up? This
> sounds like something that can cause more problems than it was
> reporting on.

It's going to get NMI in exactly the same situations it does with the 
current arch_trigger_all_cpu_backtrace(), the only difference being that 
it doesn't try to invoke printk() from inside NMI. The IPI-NMI is used 
solely as a point of synchronization for the stack dumping.

> Then you also need to print out the data while the NMIs still spin.

Exactly, that's actually the whole point.

Thanks,

-- 
Jiri Kosina
SUSE Labs
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