linux-kernel - Re: [PATCH] printk, allow different timestamps for printk.time [v2]

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Message-ID: <20160204164824.GB3305@pathway.suse.cz>
Date:	Thu, 4 Feb 2016 17:48:24 +0100
From:	Petr Mladek <pmladek@...e.com>
To:	Prarit Bhargava <prarit@...hat.com>
Cc:	linux-kernel@...r.kernel.org, John Stultz <john.stultz@...aro.org>,
	Xunlei Pang <pang.xunlei@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Baolin Wang <baolin.wang@...aro.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Tejun Heo <tj@...nel.org>,
	Peter Hurley <peter@...leysoftware.com>,
	Vasily Averin <vvs@...tuozzo.com>,
	Joe Perches <joe@...ches.com>
Subject: Re: [PATCH] printk, allow different timestamps for printk.time [v2]

On Thu 2016-01-28 07:43:49, Prarit Bhargava wrote:
> +static u64 printk_get_ts(void)
> +{
> +	u64 mono, offset_real;
> +
> +	if (printk_time == 0)
> +		return 0;
> +
> +	if (printk_time == 1)
> +		return local_clock();
> +
> +	mono = ktime_get_log_ts(&offset_real);
> +
> +	if (printk_time == 2)
> +		return mono;
> +
> +	return mono + offset_real;

At least dmesg is not capable to read the absolute size of the
real time. It expects offset against the start of the timekeeping
stuff or so. I get this:

$> dmesg | tail -n 5
[    7.128924] IPv6: ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready
[    0.000000] printk: timestamp set to 0.
[  179.983704] printk: timestamp set to 1.
[  181.895655] printk: timestamp set to 2.
[1454602412.026424] printk: timestamp set to 3.

$dmesg -T -S | tail -n 5
[Thu Feb  4 17:10:34 2016] IPv6: ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready
[Thu Feb  4 17:10:27 2016] printk: timestamp set to 0.
[Thu Feb  4 17:13:26 2016] printk: timestamp set to 1.
[Thu Feb  4 17:13:28 2016] printk: timestamp set to 2.
[Fri Mar 10 09:23:59 2062] printk: timestamp set to 3.

Please, note that the last entry points to the year 2062.


> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -13,7 +13,7 @@ config PRINTK_TIME
>  	  be included, not that the timestamp is recorded.
>  
>  	  The behavior is also controlled by the kernel command line
> -	  parameter printk.time=1. See Documentation/kernel-parameters.txt
> +	  parameter printk.time=[0-3]. See Documentation/kernel-parameters.txt

Please, mention there the mention of the numbers. The cross reference
is annoying if you look at the help when configuring kernel build.

Also there is a mismatch between the values and the type of
CONFIG_PRINTK_TIME. You should change it to

	int "Show timing information on printks"
	range 0 3

Then you need to update the default value for all
architectures in all arch/*/configs/*_defconfig

It seems that it is enabled in most architectures. I would suggest
to define

      default "1"

in lib/Kconfig.debug. Then you could remove the definition from
most defconfigs. The advantage is that even "make oldconfig"
will propose reasonable default.

Best Regards,
Petr

PS: Please, put version number to the subject, e.g. [PATCH v4].
It makes it slightly easier to orientate in the variants ;-)