linux-kernel - Re: [PATCH] ext4: Add a stub for mpage_da

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Date:	Wed, 30 Sep 2009 10:20:49 -0400
From:	Theodore Tso <tytso@....edu>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Josh Stone <jistone@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ext4: Add a stub for mpage_da_data in the trace header

On Wed, Sep 30, 2009 at 09:33:35AM -0400, Christoph Hellwig wrote:
> On Tue, Sep 29, 2009 at 02:40:07PM -0700, Josh Stone wrote:
> > The tracepoint ext4_da_write_pages has a struct mpage_da_data*
> > parameter, but that struct is only defined in fs/ext4/ext4.h.  This
> > patch adds a forward declaration for that struct, so this tracepoint
> > header can still be used by tools like SystemTap.
> 
> That's not what the tracepoints are for anyway.  No hacks for out of
> tree crap like this please - just use the trace buffer directly like
> ftrace and then you don't actually need any data types.
> 

Josh, you do realize that SystemTap can pull stuff out of the trace
buffer by examining information found in 

	/sys/kernel/debug/tracing/events/*/*/format

right?   For example:

name: ext4_sync_fs
ID: 600
format:
	field:unsigned short common_type;	offset:0;	size:2;
	field:unsigned char common_flags;	offset:2;	size:1;
	field:unsigned char common_preempt_count;	offset:3;	size:1;
	field:int common_pid;	offset:4;	size:4;
	field:int common_tgid;	offset:8;	size:4;

	field:dev_t dev;	offset:12;	size:4;
	field:int wait;	offset:16;	size:4;

print fmt: "dev %s wait %d", jbd2_dev_to_name(REC->dev), REC->wait

No need to for users to download gigabytes and gigabytes of DWARF
crapola (and for developers to grow old waiting for a kernel compile
with -g enabled to complete); SystemTap can just get what it needs
straight out of /sys/kernel/debug/tracing/events, and then reading
what it needs out of the ring buffer.

In any case I've added the blind structure pointer to
include/trace/events/ext4.h, but in the long term you're *much* better
off pulling the information you need from the ftrace ring buffer and
getting the information you need to interpret it out of
/sys/kernel/debug/tracing.

     	      				- Ted
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