linux-kernel - CONFIG_NO_HZ breaks blktrace timestamps

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Message-id: <1199918912.8388.13.camel@lap75545.ornl.gov>
Date:	Wed, 09 Jan 2008 17:48:32 -0500
From:	David Dillow <dillowda@...l.gov>
To:	linux-btrace@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org
Subject: CONFIG_NO_HZ breaks blktrace timestamps

While trying to gain some insight into a disk issue, I found that
blktrace/blkparse was giving me bogus traces -- I was seeing requests
complete before they were even dispatched or queued even! I had thought
that maybe this was an issue with SMP on the box, but when running with
'maxcpus=1', it told me that my 53 second test run only took 3.5
seconds.

I started tracking this down, and upon looking at cpu_clock(), and found
that it uses sched_clock(), which is based on jiffies. At this point I
had an ahah! moment and remembered that I had NO_HZ enabled.

Turning that off got blktrace to report the proper time for the test
run.

I'm not sure of the proper fix, nor am I sure I'm going to have much
time to track this down at the momenet, but I thought someone should
know.

-- 
Dave Dillow
National Center for Computational Science
Oak Ridge National Laboratory
(865) 241-6602 office

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