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

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Message-ID: <20130408110112.GA8332@thunk.org>
Date:	Mon, 8 Apr 2013 07:01:12 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	"Frank Ch. Eigler" <fche@...hat.com>
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

On Sun, Apr 07, 2013 at 05:59:06PM -0400, Frank Ch. Eigler wrote:
> > 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?

Is there any reason why the error message couldn't be simplified, to
something as "kernel symbol not found"?  I wasn't sure if the problem
was that there was some incompatibility between a recent change with
kprobe and systemtap, or parse failure in the systemtap script, etc.

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

If the goal is ease of use, I suspect the more important thing that
systemtap could do is to make its error messages more easily
understandable, instead of pointing the user to read a man page where
the user then has to figure out which one of a number of failure
scenarios were caused by a particularly opaque error message.  (The
man page doesn't even say that "semantic error while resolving probe
point" means that a kernel function doesn't exist -- especially
complaining about the kprobe identifier points the user in the wrong
direction.)

							- Ted
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