Re: [PATCH v3 2/5] dax: add tracepoint infrastructure, PMD tracing

[Steven Rostedt]

On Thu,  1 Dec 2016 09:37:48 -0700
Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx> 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 type of mapping, the
> fault flags, 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-1441  [005] ....    32.582758: xfs_filemap_pmd_fault: dev 259:0 ino
> 0x1003
> 
> big-1441  [005] ....    32.582776: dax_pmd_fault: dev 259:0 ino 0x1003
> shared WRITE|ALLOW_RETRY|KILLABLE|USER address 0x10505000 vm_start
> 0x10200000 vm_end 0x10700000 pgoff 0x200 max_pgoff 0x1400
> 
> big-1441  [005] ....    32.583292: dax_pmd_fault_done: dev 259:0 ino 0x1003
> shared WRITE|ALLOW_RETRY|KILLABLE|USER address 0x10505000 vm_start
> 0x10200000 vm_end 0x10700000 pgoff 0x200 max_pgoff 0x1400 NOPAGE
> 
> Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> Suggested-by: Dave Chinner <david@xxxxxxxxxxxxx>
> Reviewed-by: Jan Kara <jack@xxxxxxx>

Acked-by: Steven Rostedt <rostedt@xxxxxxxxxxx>

-- Steve