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From:   Prarit Bhargava <prarit@redhat.com>
To:     corbet@lwn.net, pmladek@suse.com, rostedt@goodmis.org,
        sergey.senozhatsky@gmail.com
Cc:     chunyan.zhang@unisoc.com, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, orsonzhai@gmail.com,
        zhang.lyra@gmail.com
Subject: Re: [PATCH V13] printk: Add monotonic, boottime, and realtime timestamps
Date:   Wed, 29 Jul 2020 08:22:36 -0400
Message-Id: <20200729122236.17418-1-prarit@redhat.com>
In-Reply-To: <20200729114423.30606-1-zhang.lyra@gmail.com>
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  Chunyan Zhang <zhang.lyra@gmail.com> wrote:
> From: Prarit Bhargava <prarit@redhat.com>
> 
> 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 timestamps by adding options for no
> timestamp, the local hardware clock, the monotonic clock, the boottime
> 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.
> 
> v13: This patch seems have being forgotten for 3 years. Rebase it on
> the latest kernel v5.8, reolve conflicts and fix compiling errors.
> Change code to adapt new printk_time usage.
> Petr's concern on printk_time is addressed by current version of kernel, too.

Lyra,

Copying a reply I sent to Orson who sent me this patch privately this
morning with some additional information.

ISTR the reason that this was dropped was because of the a problem with
the way systemd read the kernel's timestamps.  It got the attention of
Linus, and it was then pulled from the tree.

I need to go back and review the entire thread as it's been several years
since we had the discussion although ISTR someone mentioning that doing two
timestamps would not be a problem for systemd.

For example,

[48551.015086]

would be

[48551.015086] m[xxxx.xxxx]

for the monotonic clock timestamp, and

[48551.015086] b[xxxx.xxxx]

for the boottime clock, etc.

P.