linux-kernel - Re: [PATCH] printk: Print cpu number along with time

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Message-ID: <alpine.DEB.2.02.1404241518210.24983@chino.kir.corp.google.com>
Date:	Thu, 24 Apr 2014 15:18:52 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Jiri Kosina <jkosina@...e.cz>
cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jörn Engel <joern@...fs.org>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	peterz@...radead.org, akpm@...ux-foundation.org, cxie@...hat.com,
	Jiri Slaby <jslaby@...e.cz>
Subject: Re: [PATCH] printk: Print cpu number along with time

On Fri, 25 Apr 2014, Jiri Kosina wrote:

> > > +#ifdef CONFIG_PRINTK_CPU
> > > +	if (!buf)
> > > +		return snprintf(NULL, 0, "[%5lu.000000,%02x] ",
> > 
> > %02x for a cpu?  What happens on machines with 8k cpus?
> 
> Ummm ... what issue do you see here, Greg? It'll print 0x1f40, no?
> 

I think he's referring to the alignment with %02x.

> > And is this really an issue?  Debugging by using printk is fun, but not 
> > really something that people need to add a cpu number to.  Why not just 
> > use a tracepoint in your code to get the needed information instead?
> 
> Well, if you have dmesg dump from panic that happens every other year, and 
> you have to do post-mortem analysis on it, I am pretty sure you would love 
> to be able to figure out how the stack traces would look like without 
> inter-CPU interleaving. And I am pretty sure you wouldn't want to 
> insert/enable a tracepoint and wait another two years for the bug to 
> trigger again.
> 

Sounds like the appropriate fix would be to serialize stack dumping to the 
kernel log.
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