linux-kernel - Re: [PATCH v2] printk: Add boottime and real timestamps

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Date:   Tue, 1 Aug 2017 13:35:19 -0400
From:   Prarit Bhargava <prarit@...hat.com>
To:     John Stultz <john.stultz@...aro.org>
Cc:     lkml <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>,
        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>,
        Peter Zijlstra <peterz@...radead.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>,
        Josh Poimboeuf <jpoimboe@...hat.com>, linux-doc@...r.kernel.org
Subject: Re: [PATCH v2] printk: Add boottime and real timestamps



On 08/01/2017 01:00 PM, John Stultz wrote:
> On Tue, Aug 1, 2017 at 5:55 AM, Prarit Bhargava <prarit@...hat.com> wrote:
>> printk.time=1/CONFIG_PRINTK_TIME=1 adds a unmodified local hardware clock
>> timestamp to printk messages.  The local hardware clock loses time each
>> day making it difficult to determine exactly when an issue has occurred in
>> the kernel log, and making it difficult to determine how kernel and
>> hardware issues relate to each other in real time.
>>
>> Make printk output different timestampes by adding options for no
>> timestamp, the local hardware clock, the monotonic clock, and the real
>> clock.  Allow a user to pick one of the clocks by using the printk.time
>> kernel parameter.  Output the type of clock in
>> /sys/module/printk/parameters/time so userspace programs can interpret the
>> timestamp.
>>
>> Real clock & 32-bit systems:  Selecting the real clock printk timestamp may
>> lead to unlikely situations where a timestamp is wrong because the real time
>> offset is read without the protection of a sequence lock in the call to
>> ktime_get_log_ts() in printk_get_ts().
>>
>> v2: Use peterz's suggested Kconfig options.  Merge patchset together.  Fix
>> i386 !CONFIG_PRINTK builds.
>>
>> Signed-off-by: Prarit Bhargava <prarit@...hat.com>
> ...
>> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
>> index 98fe715522e8..7a8870b4ddbb 100644
>> --- a/lib/Kconfig.debug
>> +++ b/lib/Kconfig.debug
>> @@ -1,8 +1,39 @@
>>  menu "printk and dmesg options"
>>
>> +choice
>> +       prompt "printk default clock"
>> +       config PRINTK_TIME_DISABLE
>> +       bool "Disabled"
>> +       help
>> +        Selecting this option disables the time stamps of printk().
>> +
>> +       config PRINTK_TIME_LOCAL
>> +       bool "Local Clock"
>> +       help
>> +         Selecting this option causes the time stamps of printk() to be
>> +         stamped with the unadjusted hardware clock.
>> +
>> +       config PRINTK_TIME_MONO
>> +       bool "CLOCK_MONOTONIC"
>> +       help
>> +         Selecting this option causes the time stamps of printk() to be
>> +         stamped with the adjusted monotonic clock.
>> +
>> +       config PRINTK_TIME_REAL
>> +       bool "CLOCK_REALTIME"
>> +       help
>> +         Selecting this option causes the time stamps of printk() to be
>> +         stamped with the adjusted realtime clock.
> 
> Its been asked already, but I've not yet seen an answer.

Sorry for missing this.

> Is there a reason your not also adding PRINTK_TIME_BOOT here (which to
> me would be more generally useful then REAL or MONO)?

REAL has been useful to me in debug cases where events on the system were timed
to the wall clock (ex cron job running at 3AM).  I hadn't really thought much
about using BOOT TBH because MONO seemed to work just fine.

Mark Salyzyn, did you want BOOT or MONO?

P.