From: "Jose R. Santos" <jrs@us.ibm.com>
Subject: Re: tracepoints in ext4 (and/or ext3?)
Date: Tue, 12 Aug 2008 08:19:54 -0500
Message-ID: <20080812081954.5f5eeb10@gara>
References: <48A09E7F.7060605@redhat.com>
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Cc: ext4 development <linux-ext4@vger.kernel.org>
To: Eric Sandeen <sandeen@redhat.com>
In-Reply-To: <48A09E7F.7060605@redhat.com>
Sender: linux-ext4-owner@vger.kernel.org

On Mon, 11 Aug 2008 15:18:07 -0500
Eric Sandeen <sandeen@redhat.com> wrote:

> As just an initial inquiry, I'm wondering how people would feel about
> putting some tracepoints (trace_mark()) into ext[34] for monitoring the
> fs behavior.
> 
> Good/bad/indifferent?
> 
> Thanks,
> 
> -Eric

Good idea, although Im not sure if ext[34] is the best place we should
start putting markers though.

For ext[34], I would start by putting markers on VFS entry points into
ext4 and journal activity.  For these to be useful though, we also need
markers in the following places:

iochedulers
	elv_next_request()
	elv_add_request()
	elv_completed_request()

scsi
	scsi_prep_fn()
	scsi_dispatch_cmd()
	scsi_done()
	scsi_io_completion()

Entry and exit points of all IO system calls.
	AND
The VFS call for these system calls.

bio
	generic_make_request()
	bio_endio()

Scheduler
	idle_balance()  Personally, I find it useful to know when a
			machine goes idle because is stalling on IO


Im sure Im missing something but this should be a good start to be able
to track the life of a pending IO to see where deficiencies lie.

-JRS