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SubjectRe: [PATCH 3/3 v11] printk: Add monotonic, boottime, and realtime timestamps
On 09/15/2017 06:28 AM, Petr Mladek wrote:
> I am still slightly nervous that external tools would need updating.
> Also they might have troubles to interpret the time stamps especially
> when the source is changed at runtime via
> /sys/module/printk/parameters/time.
My comment below is a rehash/summary:

In the discussion, it appears that DAC protection is enough to prevent
flippant changes. The use cases I can imagine for runtime alteration
fall in two groups, late boot changes after all disks are mounted and
the application layers have started; or as an aid to debugging where the
deliberate nature can be accounted for. Change it, erase the logs is the
KISS solution, so the tools do not have to 'sniff' the stream for
dynamic changes, likely getting the 'leader' wrong, checking
/sys/module/printk/parameters/time for the current/last timebase.

To mitigate the 'leader' issue, or post-mortem/off-machine
interpretation, really for the debugging corner case IMHO, I had
proposed that local, and perhaps monotonic, time print as-is as they are
almost(?) imperceptibly different, but that realtime add a U suffix (to
denote time is UTC), and that boottime add a B suffix (well, because) so
that tools can discern. Monotonic could have a M suffix if it is really
a stickler. This proposal would require more disruptive tool
modifications and should be scoped as a separate effort. I do expect a
debate regarding upper and lower case ...

I have a patch waiting in the wings here where disruptive time changes
(suspend/resume/hibernate/restore; maybe date(1), ntpd(8) or embedded
systems LTE hardware time updates) will report dual timestamps so that
resynchronization and tracking can happen in post-mortem on the stream,
I expect to use the above proposal for the 'second' occasional timestamp.

-- Mark

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