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    SubjectRe: /proc/stat btime accuracy problem
    On Wed, Jun 1, 2011 at 5:58 PM, john stultz <johnstul@us.ibm.com> wrote:
    > On Wed, 2011-06-01 at 17:35 -0600, Bjorn Helgaas wrote:
    >> On Wed, Jun 1, 2011 at 4:35 PM, john stultz <johnstul@us.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).
    >
    > Huh.
    >
    > So yea, if we are very early at boot, we're likely using the jiffies
    > clocksource, which is basically a software-based tick counter, which
    > would be prone to lost-ticks issues if irqs were disabled for too long.
    >
    > Do you know if this is this a relatively new issue?
    >
    > My first instinct is "don't do that!" to whatever driver is disabling
    > irqs for so long. Do you know what's actually causing these long irq off
    > periods?
    >
    > I assume you're noticing this offset by seeing that CLOCK_REALTIME is
    > off from the RTC right after boot? How severe is this? The RTC read is
    > only second granular, so there's a fair amount of error (~1 second)
    > possible right at boot, so this then must be many seconds worth of lost
    > ticks to be noticeable, right?

    I'm using 2.6.34, so not really new. I think the major offender is
    kernel serial printk, which is done in polled mode. A *lot* of it,
    e.g., 30+ seconds' worth. I wonder if there's some reasonably clean
    way to resync with the RTC, say at the time we register a clocksource
    better than jiffies, or in clocksource_done_booting(), or something.

    Bjorn
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