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

[Jan Kara]

On Wed 23-11-16 11:44:19, 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
> 
> Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> Suggested-by: Dave Chinner <david@xxxxxxxxxxxxx>

Looks good. Just one minor comment:

> +	TP_printk("%s mapping %s address %#lx vm_start %#lx vm_end %#lx "
> +		"pgoff %#lx max_pgoff %#lx %s",
> +		__entry->vm_flags & VM_SHARED ? "shared" : "private",
> +		__entry->flags & FAULT_FLAG_WRITE ? "write" : "read",
> +		__entry->address,
> +		__entry->vm_start,
> +		__entry->vm_end,
> +		__entry->pgoff,
> +		__entry->max_pgoff,
> +		__print_flags(__entry->result, "|", VM_FAULT_RESULT_TRACE)
> +	)
> +)

I think it may be useful to dump full 'flags', not just FAULT_FLAG_WRITE...

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR