linux-kernel - Re: strange timestamp in dmesg

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Date:	Fri, 6 Jun 2008 11:25:47 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	"Denys Fedoryshchenko" <denys@...p.net.lb>,
	linux-kernel@...r.kernel.org
Subject: Re: strange timestamp in dmesg

On Fri, 06 Jun 2008 15:49:26 +0200 Andi Kleen <andi@...stfloor.org> wrote:

> Andrew Morton <akpm@...ux-foundation.org> writes:
> 
> >> supports DPO and FUA
> >> [    9.801761] sd 1:0:1:0: [sdc] 143374744 512-byte hardware sectors (73408 MB)
> >> [    9.673388] sd 1:0:1:0: [sdc] Write Protect is off
> >> [    9.673395] sd 1:0:1:0: [sdc] Mode Sense: ab 00 10 08
> >> [    9.806210] sd 1:0:1:0: [sdc] Write cache: enabled, read cache: enabled,
> >> supports DPO and FUA
> >> [    9.806220]  sdc: sdc1
> >> [    9.682136] sd 1:0:1:0: [sdc] Attached SCSI disk
> >> [   13.786405] SGI XFS with large block numbers, no debug enabled
> >> [   13.633457] XFS mounting filesystem sdb2
> >> [   13.724345] Starting XFS recovery on filesystem: sdb2 (logdev: internal)
> >> [   14.251356] Ending XFS recovery on filesystem: sdb2 (logdev: internal)
> >> [   15.379298] XFS mounting filesystem sdc1
> >> [   15.468255] Starting XFS recovery on filesystem: sdc1 (logdev: internal)
> >> [   14.514314] Ending XFS recovery on filesystem: sdc1 (logdev: internal)
> >> [   14.767260] warning: `squid' uses 32-bit capabilities (legacy support in use)
> >> [   17.589751] e1000: eth0: e1000_watchdog: NIC Link is Up 1000 Mbps Full
> >> Duplex, Flow Control: RX/TX
> >> 
> >
> > whoa, that's weird.  We've seen timestamps jump forward a single hop of
> > ~100000 seconds, but that's all over the place.
> 
> No it's expected since printk uses sched_clock() and sched clock is not synchronous
> between CPUs on systems without synchronized/invariant TSC (like Opteron)
> All sched_clock() users are expected to handle it.

We've seen a storm of hey-my-timestamps-went-weird reports in just the
past month or so.  I don't recall it being (such) a problem before that.

Did we change something?

> I always advocated just always using jiffies for printk. The only drawback would
> be that it won't increase in interrupt off sections, but if you have
> one that is longer than a jiffie then you have enough other problems.

I forget why, but we _were_ going to have an (arch-overrideable)
printk_clock() function.  And we still could.  The x86 implementation
of that could fall back to jiffies if the TSCs are out of whack?

<googles>

In fact it looks like we _did_ have a printk_clock(), only someone stole it.
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