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

From: David Rientjes <rientjes@google.com>
To: Jiri Kosina <jkosina@suse.cz>
Cc: "Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	"Jörn Engel" <joern@logfs.org>, "Rik van Riel" <riel@redhat.com>,
	linux-kernel@vger.kernel.org, peterz@infradead.org,
	akpm@linux-foundation.org, cxie@redhat.com,
	"Jiri Slaby" <jslaby@suse.cz>
Subject: Re: [PATCH] printk: Print cpu number along with time
Date: Thu, 24 Apr 2014 15:18:52 -0700 (PDT)	[thread overview]
Message-ID: <alpine.DEB.2.02.1404241518210.24983@chino.kir.corp.google.com> (raw)
In-Reply-To: <alpine.LNX.2.00.1404250005020.8903@pobox.suse.cz>

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.