linux-kernel - Re: [PACTH v1] trace: Add trace events for open(), exec() and uselib()

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Message-ID: <20160801163156.704a48c7@gandalf.local.home>
Date:	Mon, 1 Aug 2016 16:31:56 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Robert Foss <robert.foss@...labora.com>
Cc:	Al Viro <viro@...IV.linux.org.uk>, mingo@...hat.com,
	scott@...ntu.com, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PACTH v1] trace: Add trace events for open(), exec() and
 uselib()

On Mon, 1 Aug 2016 15:51:54 -0400
Robert Foss <robert.foss@...labora.com> wrote:

> On 2016-08-01 02:10 PM, Al Viro wrote:
> > On Mon, Aug 01, 2016 at 01:25:40PM -0400, robert.foss@...labora.com wrote:  
> >> From: Scott James Remnant <scott@...ntu.com>
> >>
> >> This patch uses TRACE_EVENT to add tracepoints for the open(),
> >> exec() and uselib() syscalls so that ureadahead can cheaply trace
> >> the boot sequence to determine what to read to speed up the next.  
> >
> > NAK.  No Tracepoints In VFS.  Not going to happen - any tracepoint can all too
> > easily become a cast-in-stone userland ABI.
> >  
> 
> Hey Al,
> 
> I'm slightly unfamiliar with this territory, so please forgive my lack 
> of knowledge of this topic.
> 
> What is the negative side of having tracepoint be a permanent fixture in 
> the VFS ABI?

Well, tracepoints are not vetted like syscalls are, but since they are
an interface for userspace tools, they can become just as permanent
of a fixture without the forethought of it being something that must be
maintained forever. This has already bitten us a couple of times.

> And how is VFS different from other subsystems in that regard?

The main difference is that Al is the maintainer of VFS and he's the
one that has to deal with maintaining a permanent tracepoint. He could
give less about other subsystems because it ain't his problem ;-)

-- Steve