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

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Message-ID: <20140424195821.GA3092@kroah.com>
Date:	Thu, 24 Apr 2014 12:58:21 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Jörn Engel <joern@...fs.org>
Cc:	Jiri Kosina <jkosina@...e.cz>, 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, Apr 24, 2014 at 03:40:24PM -0400, Jörn Engel wrote:
> On Wed, 23 April 2014 20:52:47 -0400, Jörn Engel wrote:
> > 
> > I use the patch below for some time now.  While it doesn't avoid the
> > log pollution in the first place, it lessens the impact somewhat.
> 
> Added a config option and ported it to current -linus.  Andrew, would
> you take this patch?
> 
> ---
> 
> Sometimes the printk log is heavily interleaving between different cpus.
> This is particularly bad when you have two backtraces at the same time,
> but can be annoying in other cases as well.  With an explicit cpu
> number, a simple grep can disentangle the mess for you.
> 
> Signed-off-by: Joern Engel <joern@...fs.org>
> ---
>  kernel/printk/printk.c | 19 ++++++++++++++++---
>  lib/Kconfig.debug      |  9 +++++++++
>  2 files changed, 25 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index a45b50962295..b9e464924825 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -200,6 +200,7 @@ struct printk_log {
>  	u16 len;		/* length of entire record */
>  	u16 text_len;		/* length of text buffer */
>  	u16 dict_len;		/* length of dictionary buffer */
> +	u16 cpu;		/* cpu the message was generated on */
>  	u8 facility;		/* syslog facility */
>  	u8 flags:5;		/* internal record flags */
>  	u8 level:3;		/* syslog level */
> @@ -346,6 +347,7 @@ static void log_store(int facility, int level,
>  	msg->facility = facility;
>  	msg->level = level & 7;
>  	msg->flags = flags & 0x1f;
> +	msg->cpu = smp_processor_id();
>  	if (ts_nsec > 0)
>  		msg->ts_nsec = ts_nsec;
>  	else
> @@ -859,7 +861,7 @@ static bool printk_time;
>  #endif
>  module_param_named(time, printk_time, bool, S_IRUGO | S_IWUSR);
>  
> -static size_t print_time(u64 ts, char *buf)
> +static size_t print_time(u64 ts, u16 cpu, char *buf)
>  {
>  	unsigned long rem_nsec;
>  
> @@ -868,11 +870,20 @@ static size_t print_time(u64 ts, char *buf)
>  
>  	rem_nsec = do_div(ts, 1000000000);
>  
> +#ifdef CONFIG_PRINTK_CPU
> +	if (!buf)
> +		return snprintf(NULL, 0, "[%5lu.000000,%02x] ",

%02x for a cpu?  What happens on machines with 8k cpus?

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?

thanks,

greg k-h
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