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Date: Wed, 16 Apr 2014 19:44:44 -0400
From: Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH 2/2] xfs: Nuke XFS_ERROR macro
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To: Dave Chinner <david@fromorbit.com>
Cc: Christoph Hellwig <hch@infradead.org>, Eric Sandeen <sandeen@sandeen.net>, xfs-oss <xfs@oss.sgi.com>

On Thu, 17 Apr 2014 08:08:08 +1000
Dave Chinner <david@fromorbit.com> wrote:

 
> > Here's the "best" I've come up with so far...
> > 
> > # for FUNCTION in `grep "t xfs_" /proc/kallsyms | awk '{print $3}'`; do echo "r:ret_$FUNCTION $FUNCTION \$retval" >> /sys/kernel/debug/tracing/kprobe_events; done

BTW, it's usually better to do:

grep 't xfs_' /proc/kallsyms | awk '{print $3}' ; while read FUNCTION; do ....

> > 
> > # for ENABLE in /sys/kernel/debug/tracing/events/kprobes/ret_xfs_*/enable; do echo 1 > $ENABLE; done
> > 
> > run a test that fails:
> > 
> > # dd if=/dev/zero of=newfile bs=513 oflag=direct
> > dd: writing `newfile': Invalid argument
> > 
> > # for ENABLE in /sys/kernel/debug/tracing/events/kprobes/ret_xfs_*/enable; do echo 0 > $ENABLE; done
> > 
> > # cat /sys/kernel/debug/tracing/trace
> > <snip>
> >            <...>-63791 [000] d... 705435.568913: ret_xfs_vn_mknod: (xfs_vn_create+0x13/0x20 [xfs] <- xfs_vn_mknod) arg1=0
> >            <...>-63791 [000] d... 705435.568913: ret_xfs_vn_create: (vfs_create+0xdb/0x100 <- xfs_vn_create) arg1=0
> >            <...>-63791 [000] d... 705435.568918: ret_xfs_file_open: (do_dentry_open+0x24e/0x2e0 <- xfs_file_open) arg1=0
> >            <...>-63791 [000] d... 705435.568934: ret_xfs_file_dio_aio_write: (xfs_file_aio_write+0x147/0x150 [xfs] <- xfs_file_dio_aio_write) arg1=ffffffffffffffea
> > 
> > Hey look, it's "-22" in hex!  ;)
> > 
> > so it's possible, but bleah.
> 
> Steve, we want to be able to trap specific return codes from
> functions. Say, for example, the first function that returns
> EINVAL/-EINVAL in XFS under a given workload.
> 
> What's the most efficient way to do that with ftrace?
> 
> And can that be set up as a trigger so we can use it to dump a
> snapshot of the last N events into the trace buffer or do other
> interesting things with that event?

Well, after you do the above, you could also do a while loop to all
those events and update the filter:

echo 'arg1 > 0xffffffffffffff00' > /debug/tracing/events/kprobes/filter

Which would trace only those functions that had an error code (assuming
the error code is less than 256). You could also use the trigger files:

echo 'traceoff if arg1 > 0xffffffffffffff00' > /debug/tracing/events/kprobes/*/trigger

The above wont actually work as is, you would need to do another while
loop of trigger files and set them each individually.

-- Steve



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