linux-kernel - [RFC patch 0/7] printk: Switch to CLOCK_MONOTONIC and store extra time stamps

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Message-Id: <20171115181531.322572387@linutronix.de>
Date:   Wed, 15 Nov 2017 19:15:31 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Linus Torvalds <torvalds@...uxfoundation.org>,
        Prarit Bhargava <prarit@...hat.com>,
        Mark Salyzyn <salyzyn@...roid.com>,
        Petr Mladek <pmladek@...e.com>, Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joe Perches <joe@...ches.com>
Subject: [RFC patch 0/7] printk: Switch to CLOCK_MONOTONIC and store extra
 time stamps

Following up on yesterdays discussion about adding support for reliably
correlatable time stamps to the printk ringbuffer entries, I split up that
draft patch from yesterday, addressed review feedback and actually tested
it.

There are a few things which are visible changes:

1) The fine grained time stamps start later in the boot process because
   local_clock() is initialized before a high resolution clocksource is
   installed.

2) During suspend/resume between timekeeping_suspend() and
   timekeeping_resume() the NMI safe accessors return a stale timestamp,
   i.e. the timestamp which was captured in timekeeping_suspend()

Mitigation:

 #1 Can be mitigated by a horrible hack, which I added as last patch in the
    series. It uses local_clock() in early boot up to the point where a
    proper clocksource providing fine grained CLOCK_MONOTONIC is
    available. That's actually not that bad, because time won't drift apart.

 #2 Can be mitigated at least for clocksources which are safe to access past
    timekeeping_suspend(). There are patches which enable that for TSC when
    the TSC_NONSTOP_SUSPEND cpu feature bit is enabled.

    For clocksources which cannot be accessed past timekeeping_suspend()
    the workaround #1 can be used (see last patch^Whack). Though this might
    cause time inconsistencies (non monotonic time stamps).

Not sure if any of this is a real problem, because not all
local/sched_clock() implementations are functional past
timekeeping_suspend(). 

I left all 3 time stamps in the patch for now so people can actually play
with that.

Thanks,

	tglx