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

From: Dave Chinner <david@fromorbit.com>
To: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: linux-kernel@vger.kernel.org,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Andrew Morton <akpm@linux-foundation.org>,
	Christoph Hellwig <hch@lst.de>,
	Dan Williams <dan.j.williams@intel.com>,
	Ingo Molnar <mingo@redhat.com>, Jan Kara <jack@suse.cz>,
	Matthew Wilcox <mawilcox@microsoft.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-mm@kvack.org, linux-nvdimm@ml01.01.org
Subject: Re: [PATCH 3/6] dax: add tracepoint infrastructure, PMD tracing
Date: Fri, 25 Nov 2016 14:00:59 +1100	[thread overview]
Message-ID: <20161125030059.GY31101@dastard> (raw)
In-Reply-To: <1479926662-21718-4-git-send-email-ross.zwisler@linux.intel.com>

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"'...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com