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

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Date:   Tue, 1 Aug 2017 10:00:30 -0700
From:   John Stultz <john.stultz@...aro.org>
To:     Prarit Bhargava <prarit@...hat.com>
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 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.
Is there a reason your not also adding PRINTK_TIME_BOOT here (which to
me would be more generally useful then REAL or MONO)?

thanks
-john