Re: [PATCH 0/2 v6] printk, Add monotonic and real printk timestamps

[Petr Mladek]

On Mon 2016-04-18 11:30:52, Prarit Bhargava wrote:
> 
> 
> On 03/10/2016 05:00 AM, Petr Mladek wrote:
> > On Tue 2016-03-08 06:03:24, Prarit Bhargava wrote:
> >>
> >>
> >> On 03/08/2016 02:59 AM, Thomas Gleixner wrote:
> >>> On Tue, 23 Feb 2016, Prarit Bhargava wrote:
> >>>
> >>>> This patchset adds monotonic and real printk timestamps.  The first patch
> >>>> changes CONFIG_PRINT_TIME from a bool to an int to allow for the additional
> >>>> timestamps that are added in patch 2.
> >>>>
> >>>> Changes from v6: Petr Mladek pointed out that the current patch
> >>>> fails to indicate to userspace programs which timestamp is being used.
> >>>
> >>> How is that solved?
> >>
> >> Hi Thomas,
> >>
> >> Userspace programs can now look at /sys/modules/printk/parameters/time which
> >> will contain [0-3] for the timestamp clock.
> > 
> > But it includes only the current setting that is valid only for
> > messages printed with this setting. The ring buffer might include
> > different messages produced with different setting.
> > 
> > I suggest to look how dmesg handles the time stamp. I wonder how it
> > converts the relative time into an absolute one. I wonder if you
> > could convert all timestamps to the relative format, so that you
> > do not need to change all userspace tools at all.
> 
> I looked into this and the only thing I can come up with is that I modify the
> patchset to allow users to set the timestamp type at boot but not runtime.
> Having to set it at runtime would require additional timestamp information be
> added to the output of /dev/kmsg.
> 
> Petr -- the dmesg code can be found in the utils-linux package,
> sys-utils/dmesg.c.  The timestamping displaying code is this function:
> 
> static struct tm *record_localtime(struct dmesg_control *ctl,
>                                    struct dmesg_record *rec,
>                                    struct tm *tm)
> {
>         time_t t = ctl->boot_time + rec->tv.tv_sec;
>         return localtime_r(&t, tm);
> }
>
> If you have any suggestion on how to modify it, I'm more than willing to do it.
>  If not, then I suggest I change the code to make the timestamp RO during runtime.

Hmm, If you allow to change the timestamp format only at boot time, it
will make things easier. I just wonder if it would work correctly for
early messages. For example, are there any messages printed before
the real time clock is initialized? Which timestamp will they use?

Also note that you still need to modify the dmesg code. It must
not add boot_time when real time timestamp is used.

And you need to modify also the other tools, e.g. crash.


Best Regards,
Petr