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

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Date:   Tue, 18 Jul 2017 16:25:11 -0700
From:   Mark Salyzyn <salyzyn@...roid.com>
To:     Thomas Gleixner <tglx@...utronix.de>
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)

[TL;DR]

On 07/18/2017 03:35 PM, Thomas Gleixner wrote:
> Can we please fix the tools and not introduce that horror in the kernel?
IMHO, we can switch this patch to epoch.ns time; but the momentum on the
tools will end up with us internally patching it back to RTC format at 
least until
we can get our act together ... I have accepted that risk, we always like to
see all changes upstream though. Would an additional config parameter
appease the situation?
> Just for the record: You cannot use that stuff deep in the suspend/resume
> code because timekeeping is suspended there as well and that call to
> getnstimeofday() is going to trigger a WARNON() when called after
> timekeeping was suspended. So please be more precise about the limitations
> of this.

Yes, I should have been clearer about that, in fact my comments in 0/4 
were _backwards_
when I reread them ;-/. One of the issues is the somewhat helpful suspend
millisecond duration requires a persistent clock, which is _not_ 
available on all
hardware. Printing the time on entry and exit from suspend and resume is 
always
possible because we are staying away from that lower level issue.

I should probably roll the details of 0/4 comments into 1/4, the intro 
has not served
its purpose.

It should also be made clearer that these single messages serve _two_ 
purposes,
the primary one is power analysis; the secondary one is log synchronization.

> 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.

-- Mark