Re: [QUESTION] printk: time namespaces and printk timestamps

[Thomas Weißschuh]

On 2026-05-25 10:37:40+0200, Petr Mladek wrote:
> On Sat 2026-05-23 17:20:45, Thomas Weißschuh wrote:
> > It looks like the monotonic timestamps of printk records do not take the
> > callers time namespace into account when emitted through /dev/kmsg,
> > /proc/kmsg and syslog(2).
> > 
> > To me this looks like a bug. What do you think? Should it be changed?
> 
> To be honest, I have never heard about time namespaces before ;-)
> 
> So, you propose to use the timestamp with the namespace-specific
> offset when a message is added from the given namespace, e.g. via:
> 
>    echo "This message is from another time space" > /dev/kmsg
> 
> Do I get it correctly?

My idea was rather to do the conversion when *reading* the message from
a time namespaced user process.

> I see two potential problems:
> 
> 1. printk() historically used a monotonic clock. I am not sure if any
>    usespace application depends on it but there is a risk of breaking
>    something.

>From the perspective of the namespaced process currently the timestamps
from printk may in fact not be CLOCK_MONOTONIC.

>    Anyway, the printk messages are used to debug kernel problems.
>    And the order of events might be important. IMHO, messages with
>    random offsets might create confusion.

That would not be an issue when doing the conversion during read.
Each process sees the same order of events with the same relative
time increments between them. Only the absolute values would change.

Note: The offsets can never change for a given namespace or process.

> 2. printk() has to work in any context, including NMI. It requires
>    a lockless way to get the timestamps. I am not sure if the
>    namespace timestamps might be read a lockless way and if
>    it would be worth the effort.

As above.


Thomas