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SubjectRe: [PATCH v2] printk: Add boottime and real timestamps
On 08/01/2017 10:35 AM, Prarit Bhargava wrote:
> On 08/01/2017 01:00 PM, John Stultz wrote:
>> On Tue, Aug 1, 2017 at 5:55 AM, Prarit Bhargava <> 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 <>
>> ...
>>> 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"
>>> + 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.

You must IMHO include MONO (default when on?), BOOT and REAL if you are
offering the ability for the kernel to switch time base.


I had a partner request for Boottime (for kernel and userspace) for
those that are providing products that focus more on sensors. We said
no, case closed. This option in your patch would reopen that possibility.

We have too many use cases, too many partners all pulling in different
directions. We have met with resistance moving userspace to monotonic to
match kernel on phones, preference remains realtime for user space, and
monotonic for kernel. Yet watch partners preferred moving all to
monotonic time, but they required realtime (or could do with boottime)
dual-print to monitor battery and power. We only wanted to print
realtime (or monotonic depending on case) during time disruptions
(suspend/resume mainly). With your proposal we may have less resistance
to realtime for kernel and userspace, but we will have partners that
insist on using monotonic none-the-less because we do not have control,
so we _must_ provide the dual-time option for them.

The dual-time in disruptive cases depends a _lot_ on what happens with
this patch. I am expecting mine to be very sensitive to the config
settings (ie: if both select realtime, then no dual-time is printed), so
my proposal is on hiatus until this is resolved.

-- Mark

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