linux-ext4 - Re: [PATCH 3/6] dax: add tracepoint infrastructure, PMD tracing

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Message-ID: <20161125024918.GX31101@dastard>
Date:   Fri, 25 Nov 2016 13:49:18 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-nvdimm@...ts.01.org
Subject: Re: [PATCH 3/6] dax: add tracepoint infrastructure, PMD tracing

On Thu, Nov 24, 2016 at 05:32:20PM +0000, Al Viro 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.
> 
> 	It also has one hell of potential for becoming a massive nuisance.
> Keep in mind that if any userland code becomes dependent on those - that's it,
> they have become parts of stable userland ABI and are to be maintained
> indefinitely.  Don't expect "tracepoints are special case" to prevent that.

I call bullshit just like I always do when someone spouts this
"tracepoints are stable ABI" garbage.

If we want to provide stable tracepoints, then we need to /create a
stable tracepoint API/ and convert all the tracepoints that /need
to be stable/ to use it. Then developers only need to be careful
about modifying code around the /explicitly stable/ tracepoints and
we avoid retrospectively locking the kernel implementation into a
KABI so tight we can't do anything anymore....

Quite frankly, anyone that wants to stop us from
adding/removing/changing tracepoints or the code that they are
reporting information about "because ABI" can go take a long walk
off a short cliff.  Diagnostic tracepoints are not part of the
stable ABI. End of story.

> 	So treat anything you add in that manner as potential stable ABI
> you might have to keep around forever.  It's *not* a glorified debugging
> printk.

trace_printk() is the glorified debugging printk for tracing, not
trace events.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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