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

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Message-ID: <alpine.LNX.2.00.1404250005020.8903@pobox.suse.cz>
Date:	Fri, 25 Apr 2014 00:12:06 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
cc:	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 Thu, 24 Apr 2014, Greg Kroah-Hartman 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?

> 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.

-- 
Jiri Kosina
SUSE Labs
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