[ Added Linus and Al ]
On Thu, 23 May 2019 19:08:51 -0700 Alexei Starovoitov <alexei.starovoi...@gmail.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
