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From: Theodore Tso <tytso <at> MIT.EDU>
Subject: Re: [F.A.Q.] perf ABI backwards and forwards compatibility
Newsgroups: gmane.comp.emulators.kvm.devel
Date: Tuesday 8th November 2011 10:41:33 UTC (over 6 years ago)
On Nov 8, 2011, at 5:22 AM, Ingo Molnar wrote:

> We do even more than that, the perf ABI is fully backwards *and* 
> forwards compatible: you can run older perf on newer ABIs and newer 
> perf on older ABIs.

It's great to hear that!   But in that case, there's an experiment we can't
really run, which is if perf had been developed in a separate tree, would
it have been just as successful?

My belief is that perf was successful because *you* and the other perf
developers were competent developers, and who got things right.   Not
because it was inside the kernel tree.   You've argued that things were
much better because it was inside the tree, but that's not actually
something we can put to a scientific repeatable experiment.

I will observe that some of the things that caused me to be come enraged by
system tap (such as the fact that I simply couldn't even build the damned
thing on a non-Red Hat compilation environment, would not have been solved
by moving Systemtap into the kernel git tree --- at least not without
moving a large number of its external dependencies into the kernel tree as
well, such as the elf library, et. al.)   So there is a whole class of
problems that were seen in previous tooling systems that were not caused by
the fact that they were separate from the kernel, but that they weren't
being developed by the kernel developers, so they didn't understand how to
make the tool work well for kernel developers.

If we had gone back in time, and had the same set of perf developers
working in an external tree, and Systemtap and/or Oprofile had been
developed in the kernel tree, would it really have made that much
difference?   Sure, Linus and other kernel developers would have yelled at
the Systemtap and Oprofile folks more, but I haven't seen that much
evidence that they listened to us when they were outside of the kernel
tree, and it's not obvious they would have listened with the code being
inside the kernel tree.

My claim is that is that outcome wouldn't have been all that different, and
that's because the difference was *you*, Ingo Molnar, as a good engineer,
would have designed a good backwards compatible ABI whether the code was
inside or outside of the kernel, and you would have insisted on good taste
and usefulness to kernel programmers whether perf was in our out of the
kernel, and you would have insisted on kernel coding guidelines and regular
release cycles, even if perf was outside of the kernel.   As Linus
sometimes like to say, in many cases it's more about the _people_.


-- Ted

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