linux-kernel - Re: /proc/stat btime accuracy problem

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Message-ID: <BANLkTi=yEMDLEo99WZm2Bt4pi_pog5igww@mail.gmail.com>
Date:	Wed, 1 Jun 2011 17:35:22 -0600
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	john stultz <johnstul@...ibm.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: /proc/stat btime accuracy problem

On Wed, Jun 1, 2011 at 4:35 PM, john stultz <johnstul@...ibm.com> wrote:
> On Wed, 2011-06-01 at 14:50 -0600, Bjorn Helgaas wrote:
>> timekeeping_init() basically does the following:
>>
>>     xtime = RTC
>>     if (arch implements read_boot_clock())
>>         wall_to_monotonic = -read_boot_clock()
>>     else
>>       wall_to_monotonic = -xtime
>>
>> So wall_to_monotonic records some approximation of the system boot
>> time, which is then used to derive the "btime" reported in /proc/stat.
>>
>> The problem I'm seeing is that xtime is updated on timer ticks, so
>> uninterruptible code, like kernel serial printk, makes us miss ticks,
>> so xtime falls behind the RTC.
>
> Huh. So this sort of issue was common back when we had tick-based
> timekeeping (in combination with troubled hardware), but with the
> current clocksource based timekeeping, occasional lost ticks shouldn't
> really effect time.

Makes sense.  Your presentation here was a great help:
  http://sr71.net/~jstultz/tod/ols-presentation-final.pdf

> Can you explain a bit more about what kind of hardware this is happening
> on, and what clocksource is being used?

Sure.  This is an x86 box.  Normally we're using the TSC clocksource,
and I don't think the issue happens after that.  I guess my
experimentation so far has been with uninterruptible time before we
register *any* clocksource (or at least before I see any "Switching to
clocksource" messages).

Bjorn
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