netdev - Re: [RFC PATCH 00/11] bpf, trace, dtrace: DTrace BPF program type implementation and sample use

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Date:   Thu, 23 May 2019 22:40:08 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Kris Van Hees <kris.van.hees@...cle.com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, dtrace-devel@....oracle.com,
        linux-kernel@...r.kernel.org, mhiramat@...nel.org, acme@...nel.org,
        ast@...nel.org, daniel@...earbox.net, peterz@...radead.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...IV.linux.org.uk>
Subject: Re: [RFC PATCH 00/11] bpf, trace, dtrace: DTrace BPF program type
 implementation and sample use


[ Added Linus and Al ]

On Thu, 23 May 2019 19:08:51 -0700
Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:

> > > > 
> > > > I wish that was totally true, but tracepoints *can* be an abi. I had
> > > > code reverted because powertop required one to be a specific
> > > > format. To this day, the wakeup event has a "success" field that
> > > > writes in a hardcoded "1", because there's tools that depend on it,
> > > > and they only work if there's a success field and the value is 1.    
> > > 
> > > I really think that you should put powertop nightmares to rest.
> > > That was long ago. The kernel is different now.  
> > 
> > Is it?
> >   
> > > Linus made it clear several times that it is ok to change _all_
> > > tracepoints. Period. Some maintainers somehow still don't believe
> > > that they can do it.  
> > 
> > From what I remember him saying several times, is that you can change
> > all tracepoints, but if it breaks a tool that is useful, then that
> > change will get reverted. He will allow you to go and fix that tool and
> > bring back the change (which was the solution to powertop).  
> 
> my interpretation is different.
> We changed tracepoints. It broke scripts. People changed scripts.

Scripts are different than binary tools.

> 
> >   
> > > 
> > > Some tracepoints are used more than others and more people will
> > > complain: "ohh I need to change my script" when that tracepoint
> > > changes. But the kernel development is not going to be hampered by a
> > > tracepoint. No matter how widespread its usage in scripts.  
> > 
> > That's because we'll treat bpf (and Dtrace) scripts like modules (no
> > abi), at least we better. But if there's a tool that doesn't use the
> > script and reads the tracepoint directly via perf, then that's a
> > different story.  
> 
> absolutely not.
> tracepoint is a tracepoint. It can change regardless of what
> and how is using it.

Instead of putting words into Linus's mouth, I'll just let him speak
for himself. If a useful tool that reads a tracepoint breaks because we
changed the tracepoint, and Linus is fine with that. Then great, we can
start adding them to VFS and not worry about them being an ABI.

-- Steve