linux-kernel - Re: [PATCH 2/2] printk: Add boottime and real timestamps

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Date:   Fri, 28 Jul 2017 11:01:48 -0400
From:   Prarit Bhargava <prarit@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Mark Salyzyn <salyzyn@...roid.com>,
        Jonathan Corbet <corbet@....net>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...eaurora.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Christoffer Dall <cdall@...aro.org>,
        Deepa Dinamani <deepa.kernel@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Joel Fernandes <joelaf@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Olof Johansson <olof@...om.net>, Theodore Ts'o <tytso@....edu>,
        Josh Poimboeuf <jpoimboe@...hat.com>, linux-doc@...r.kernel.org
Subject: Re: [PATCH 2/2] printk: Add boottime and real timestamps



On 07/25/2017 09:00 AM, Peter Zijlstra wrote:
> On Tue, Jul 25, 2017 at 08:17:27AM -0400, Prarit Bhargava wrote:
>> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
>> index 5b1662ec546f..6cd38a25f8ea 100644
>> --- a/lib/Kconfig.debug
>> +++ b/lib/Kconfig.debug
>> @@ -1,8 +1,8 @@
>>  menu "printk and dmesg options"
>>  
>>  config PRINTK_TIME
>> -	int "Show timing information on printks (0-1)"
>> -	range 0 1
>> +	int "Show timing information on printks (0-3)"
>> +	range 0 3
>>  	default "0"
>>  	depends on PRINTK
>>  	help
>> @@ -13,7 +13,8 @@ config PRINTK_TIME
>>  	  The timestamp is always recorded internally, and exported
>>  	  to /dev/kmsg. This flag just specifies if the timestamp should
>>  	  be included, not that the timestamp is recorded. 0 disables the
>> -	  timestamp and 1 uses the local clock.
>> +	  timestamp and 1 uses the local clock, 2 uses the monotonic clock, and
>> +	  3 uses real clock.
>>  
>>  	  The behavior is also controlled by the kernel command line
>>  	  parameter printk.time=1. See Documentation/admin-guide/kernel-parameters.rst
> 
> 
> choice
> 	prompt "printk default clock"
> 	default PRIMTK_TIME_DISABLE
> 	help
> 	 goes here
> 
> 	config PRINTK_TIME_DISABLE
> 	bool "Disabled"
> 	help
> 	 goes here
> 
> 	config PRINTK_TIME_LOCAL
> 	bool "local clock"
> 	help
> 	 goes here
> 
> 	config PRINTK_TIME_MONO
> 	bool "CLOCK_MONOTONIC"
> 	help
> 	 goes here
> 
> 	config PRINTK_TIME_REAL
> 	bool "CLOCK_REALTIME"
> 	help
> 	 goes here
> 
> endchoice
> 
> config PRINTK_TIME
> 	int
> 	default 0 if PRINTK_TIME_DISABLE
> 	default 1 if PRINTK_TIME_LOCAL
> 	default 2 if PRINTK_TIME_MONO
> 	default 3 if PRINTK_TIME_REAL
> 
> 

Thanks for the above change.  I can see that makes the code simpler.

> Although I must strongly discourage using REALTIME, DST will make
> untangling your logs an absolute nightmare. I would simply not provide
> it.

I understand your concern, however, I've been in situations where REALTIME
stamping has pointed me in the direction of where a bug was.  Even with the
complicated logs I think it is worthwhile.

P.