linux-kernel - Re: [RFC patch 3/7] printk: Use clock MONOTONIC for timestamps

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Message-ID: <20171116075832.GA464@jagdpanzerIV>
Date:   Thu, 16 Nov 2017 16:58:59 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        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: Re: [RFC patch 3/7] printk: Use clock MONOTONIC for timestamps

Hello Thomas,

On (11/15/17 19:15), Thomas Gleixner wrote:
> local_clock() cannot be reliably correlated to CLOCK_MONOTONIC, which is
> used by user space, e.g. systemd, to create correlation timestamps.
> 
> There are multiple reasons:
> 
>  - CLOCK_MONOTONIC is NTP adjusted, local_clock() not. Depending on the
>    calibration accuracy and uptime significant drift can be observed.
> 
>  - CLOCK_MONOTONIC does not advance across suspend/resume for historical
>    reasons. local_clock() might or might not depending on the properties of
>    the underlying hardware counter.
> 
> Use the NMI safe accessor to clock MONOTONIC instead of local_clock(). The
> access might be slower than local_clock(), but printk is not such a
> performance critical hotpath that it matters.
> 
> Visible change:
> 
> The early boot timestamps are jiffies based longer than with local_clock()
> depending on the platform. During suspend/resume the timestamp may become
> stale when the underlying clocksource hardware is not flagged with
> CLOCKSOURCE_SUSPEND_ACCESS_OK.
> 
> A horrible follow up patch demonstrates how that could be mitigated.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
>  kernel/printk/printk.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 

a silly nitpick,

I suppose we can do

-#include <linux/sched/clock.h>
+#include <linux/timekeeping.h>

	-ss