Re: [PATCH 3/6] dax: add tracepoint infrastructure, PMD tracing

[Dave Chinner <david@fromorbit.com>]

On Mon, Nov 28, 2016 at 03:46:51PM -0700, Ross Zwisler wrote:
> On Fri, Nov 25, 2016 at 02:00:59PM +1100, Dave Chinner wrote:
> > On Wed, Nov 23, 2016 at 11:44:19AM -0700, Ross Zwisler wrote:
> > > Tracepoints are the standard way to capture debugging and tracing
> > > information in many parts of the kernel, including the XFS and ext4
> > > filesystems.  Create a tracepoint header for FS DAX and add the first DAX
> > > tracepoints to the PMD fault handler.  This allows the tracing for DAX to
> > > be done in the same way as the filesystem tracing so that developers can
> > > look at them together and get a coherent idea of what the system is doing.
> > > 
> > > I added both an entry and exit tracepoint because future patches will add
> > > tracepoints to child functions of dax_iomap_pmd_fault() like
> > > dax_pmd_load_hole() and dax_pmd_insert_mapping(). We want those messages to
> > > be wrapped by the parent function tracepoints so the code flow is more
> > > easily understood.  Having entry and exit tracepoints for faults also
> > > allows us to easily see what filesystems functions were called during the
> > > fault.  These filesystem functions get executed via iomap_begin() and
> > > iomap_end() calls, for example, and will have their own tracepoints.
> > > 
> > > For PMD faults we primarily want to understand the faulting address and
> > > whether it fell back to 4k faults.  If it fell back to 4k faults the
> > > tracepoints should let us understand why.
> > > 
> > > I named the new tracepoint header file "fs_dax.h" to allow for device DAX
> > > to have its own separate tracing header in the same directory at some
> > > point.
> > > 
> > > Here is an example output for these events from a successful PMD fault:
> > > 
> > > big-2057  [000] ....   136.396855: dax_pmd_fault: shared mapping write
> > > address 0x10505000 vm_start 0x10200000 vm_end 0x10700000 pgoff 0x200
> > > max_pgoff 0x1400
> > > 
> > > big-2057  [000] ....   136.397943: dax_pmd_fault_done: shared mapping write
> > > address 0x10505000 vm_start 0x10200000 vm_end 0x10700000 pgoff 0x200
> > > max_pgoff 0x1400 NOPAGE
> > 
> > Can we make the output use the same format as most of the filesystem
> > code? i.e. the output starts with backing device + inode number like
> > so:
> > 
> > 	xfs_ilock:            dev 8:96 ino 0x493 flags ILOCK_EXCL....
> > 
> > This way we can filter the output easily across both dax and
> > filesystem tracepoints with 'grep "ino 0x493"'...
> 
> I think I can include the inode number, which I have via mapping->host.  Am I
> correct in assuming "struct inode.i_ino" will always be the same as
> "struct xfs_inode.i_ino"?

Yes - just use inode.i_ino.

> Unfortunately I don't have access to the major/minor (the dev_t) until I call
> iomap_begin(). 

In general, filesystem tracing uses inode->sb->s_dev as the
identifier. NFS, gfs2, XFS, ext4 and others all use this.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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