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

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Message-ID: <1306975745.11492.30.camel@work-vm>
Date:	Wed, 01 Jun 2011 17:49:05 -0700
From:	john stultz <johnstul@...ibm.com>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-serial@...r.kernel.org, Alan Cox <alan@...ux.intel.com>
Subject: Re: /proc/stat btime accuracy problem

On Wed, 2011-06-01 at 18:31 -0600, Bjorn Helgaas wrote:
> On Wed, Jun 1, 2011 at 5:58 PM, john stultz <johnstul@...ibm.com> wrote:
> > 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.

CC'ing the linux-serial list and Alan for their thoughts: Does the
serial port is disabling irqs for 30+ seconds during boot seem at all
sane?

I would think it would periodically enable irqs in the polling loop to
let things be processed, but maybe not?

>   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.

I definitely think we need to address the long irq disable periods
instead of trying to hack around the issue in the timekeeping code.

thanks
-john



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