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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