linux-kernel - Re: [PATCH] clocksource, Add warning to clocksource

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Date:	Fri, 17 Oct 2014 13:23:12 -0400
From:	Prarit Bhargava <prarit@...hat.com>
To:	Prarit Bhargava <prarit@...hat.com>
CC:	linux-kernel@...r.kernel.org, John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] clocksource, Add warning to clocksource_delta() validation
 code



On 10/17/2014 09:57 AM, Prarit Bhargava wrote:
> A bug report came in against an older kernel which output "backward time"
> messages and the report noted that the upstream kernel worked.  After some
> investigation it turned out that one of the sockets was bad on the system
> and the "backward time" messages were caused by a real, but intermittent,
> hardware failure.
> 
> Commit 09ec54429c6d10f87d1f084de53ae2c1c3a81108 ("clocksource: Move
> cycle_last validation to core code") modifies the x86 clocksource such that
> if a negative delta between two reads of time is calculated the
> clocksource_delta() code will return 0.  There is no warning when this
> occurs and there really should be one in order to catch not only hardware
> issues like the issue above, but potential coding issues as the code is
> modified.  This patch introduces a WARN() which will also dump a stack
> trace to the console so the exact code path can be evaluated.
> 
> I tested this by booting on the broken hardware and left the system idle
> until a negative clocksource_delta() event occurred.
> 
> Cc: John Stultz <john.stultz@...aro.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Prarit Bhargava <prarit@...hat.com>

Sorry everyone -- I accidentally sent this out.  It is still untested and needs
some bake time.

Please ignore this patch.

P.
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