linux-kernel - Re: [PATCH v2 1/4] time: rtc-lib: Add rtc_show_time(const char *prefix

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Message-ID: <alpine.DEB.2.20.1707190913500.2286@nanos>
Date:   Wed, 19 Jul 2017 09:23:25 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Mark Salyzyn <salyzyn@...roid.com>
cc:     LKML <linux-kernel@...r.kernel.org>, rjw@...ysocki.net,
        len.brown@...el.com, pavel@....cz, linux-pm@...r.kernel.org,
        a.zummo@...ertech.it, alexandre.belloni@...e-electrons.com,
        linux-rtc@...r.kernel.org, andy.shevchenko@...il.com,
        Mark Salyzyn <salyzyn@...gle.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Thierry Strudel <tstrudel@...gle.com>,
        John Stultz <john.stultz@...aro.org>,
        Richard Cochran <richardcochran@...il.com>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Kees Cook <keescook@...omium.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Prarit Bhargava <prarit@...hat.com>
Subject: Re: [PATCH v2 1/4] time: rtc-lib: Add rtc_show_time(const char
 *prefix_msg)

On Tue, 18 Jul 2017, Mark Salyzyn wrote:
> On 07/18/2017 03:35 PM, Thomas Gleixner wrote:
> > There was some discussion about making the clock source for dmesg time
> > stamps selectable, so you can use MONOTONIC, REALTIME, BOOTTIME. The
> > patches looked sensible, but there was some showstopper vs. the user space
> > dmesg utility. See:
> 
> The timestamps are useful for the 'second' purpose of these patches when
> dmesg time is BOOTTIME or MONOTONIC, and can be turned off if REALTIME
> is selected. Having rtc_show_time a single point for switching this no doubt
> helps,
> not hinders, that dmesg issue.
> 
> The inflection points would still serve a purpose, still need
> suspend/resume/hibernate/restore. The reboot messages are _only_ useful to
> us with their timestamps, as I checked and the only tools that use those are
> for log synchronization. We may be able to do away with them on REALTIME
> dmesg'ing; but the standardization of the message as a marker would have a
> legacy purpose (!)
> 
> NB: We have a similar configuration for the user space logger, which can be
> configured to report in MONOTONIC time. We have yet to have a vendor
> use the feature, opting for REALTIME logging for user space activities.
> Our klogd (which runs at background priority and is batched) manages a
> histogram relationship between MONOTONIC and REALTIME helped by these
> prints and incorporates the REALTIME dmesg logs merged into our user
> space logging database.

There is another option to remedy this and the dmesg tooling issues:

Instead of switching the time stamps in dmesg to a different clock we might
as well have an optional secondary timestamp. So instead of:

 [  341.590930] wlan0: associated

you would get:

 [  341.590930] [ sec.usec] wlan0: associated

where the second time stamp would be CLOCK_REALTIME/BOOTTIME.

That should also solve Prarits problem, hmm?

Thanks,

	tglx