linux-kernel - Progress on system crash traces with LTTng using DAX and pmem

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Date:	Sat, 25 Oct 2014 12:51:25 +0000 (UTC)
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Matthew Wilcox <willy@...ux.intel.com>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:	lttng-dev <lttng-dev@...ts.lttng.org>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Progress on system crash traces with LTTng using DAX and pmem

Hi Matthew, Hi Ross,

A quick follow up on my progress on using DAX and pmem with
LTTng. I've been able to successfully gather a user-space
trace into buffers mmap'd into an ext4 filesystem within
a pmem block device mounted with -o dax to bypass the page
cache. After a soft reboot, I'm able to mount the partition
again, and gather the very last data collected in the buffers
by the applications. I created a "lttng-crash" program that
extracts data from those buffers and converts the content
into a readable Common Trace Format trace. So I guess
you have a use-case for your patchsets on commodity hardware
right there. :)

I've been asked by my customers if DAX would work well with
mtd-ram, which they are using. To you foresee any roadblock
with this approach ?

FYI, the main reason why my customer wants to go with a
"trace into memory that survives soft reboot" approach
rather than to use things like kexec/kdump is that they
care about the amount of time it takes to reboot their
machines. They want a solution where they can extract the
detailed crash data after reboot, after the machine is
back online, rather than requiring a few minutes of offline
time to extract the crash details.

So I guess next year I'll probably be looking into
allocating the LTTng kernel tracer buffers into an mmap'd file
within a ext2/4-DAX-over-pmem/mtd-ram filesystem. It's going
to be exciting! :)

Please keep me in CC on your next patch versions. I'm willing
to spend some more time reviewing them if needed. By the way,
do you guys have a target time-frame/kernel version you aim
at for getting this work upstream ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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