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

From: Steven Rostedt <rostedt@goodmis.org>
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>,
	Dave Chinner <david@fromorbit.com>,
	Ingo Molnar <mingo@redhat.com>, Jan Kara <jack@suse.cz>,
	Matthew Wilcox <mawilcox@microsoft.com>,
	linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
	linux-nvdimm@lists.01.org
Subject: Re: [PATCH v3 2/5] dax: add tracepoint infrastructure, PMD tracing
Date: Thu, 1 Dec 2016 11:54:43 -0500	[thread overview]
Message-ID: <20161201115443.77135c91@gandalf.local.home> (raw)
In-Reply-To: <1480610271-23699-3-git-send-email-ross.zwisler@linux.intel.com>

On Thu,  1 Dec 2016 09:37:48 -0700
Ross Zwisler <ross.zwisler@linux.intel.com> 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@linux.intel.com>
> Suggested-by: Dave Chinner <david@fromorbit.com>
> Reviewed-by: Jan Kara <jack@suse.cz>

Acked-by: Steven Rostedt <rostedt@goodmis.org>

-- Steve

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