linux-kernel - Re: Excessive stall times on ext4 in 3.9-rc2

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Message-ID: <y0mwqsehuj9.fsf@fche.csb>
Date:	Sun, 07 Apr 2013 17:59:06 -0400
From:	fche@...hat.com (Frank Ch. Eigler)
To:	"Theodore Ts'o" <tytso@....edu>
Cc:	Mel Gorman <mgorman@...e.de>, linux-ext4@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>, Jiri Slaby <jslaby@...e.cz>
Subject: Re: Excessive stall times on ext4 in 3.9-rc2


Hi -


tytso wrote:

> So I tried to reproduce the problem, and so I installed systemtap
> (bleeding edge, since otherwise it won't work with development
> kernel), and then rebuilt a kernel with all of the necessary CONFIG
> options enabled:
>
> 	CONFIG_DEBUG_INFO, CONFIG_KPROBES, CONFIG_RELAY, CONFIG_DEBUG_FS,
> 	CONFIG_MODULES, CONFIG_MODULE_UNLOAD
> [...]

That sounds about right.


> I then pulled down mmtests, and tried running watch-dstate.pl, which
> is what I assume you were using [...]

I just took a look at the mmtests, particularly the stap-fix.sh stuff.
The heroics therein are really not called for.  git kernel developers
should use git systemtap, as has always been the case.  All
compatibility hacks in stap-fix.sh have already been merged, in many
cases for months.


> [...]
> semantic error: while resolving probe point: identifier 'kprobe' at /tmp/stapdjN4_l:18:7
>         source: probe kprobe.function("get_request_wait")
>                       ^
> Pass 2: analysis failed.  [man error::pass2]
> Unexpected exit of STAP script at ./watch-dstate.pl line 296.
> I have no clue what to do next.  Can you give me a hint?

You should see the error::pass2 man page, which refers to
error::reporting, which refers to involving stap folks and running
stap-report to gather needed info.

But in this case, that's unnecessary: the problem is most likely that
the get_request_wait function does not actually exist any longer, since

commit a06e05e6afab70b4b23c0a7975aaeae24b195cd6
Author: Tejun Heo <tj@...nel.org>
Date:   Mon Jun 4 20:40:55 2012 -0700

    block: refactor get_request[_wait]()


Systemtap could endavour to list roughly-matching functions that do
exist, if you think that's be helpful.


- FChE
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