exporting INVARIANTS easily

[Warner Losh]

Greetings,

Many of the performance eating features are exported via some kind of sysctl, usually
patterned after the case of witness as debug.foo. INVARIANTS isn’t one of those
features.

https://reviews.freebsd.org/D3488

implements debug.invariants. Please comment.

I’d thought about adding it to the kern.features sysctl, but thought better of it since it
isn’t a facility that people can use.

If you include the kernel config in the kernel, you can get this information via
	config -x | grep INVARIANTS
but not all kernels do that. This is more robust.

I also know that you can load some modules compiled INVARIANTS when the base
kernel isn’t built that way and this won’t reflect that. There’s no good want to include
that information and is an uncommon use case.

Our use case? We have a raft of test machines. Most run without INVARIANTS since
we want to characterize the performance of the release under test. Some are running
INVARIANTS since we want to test the robustness as well, even at the expense of
some performance. To ensure we don’t accidentally include INVARIANTS systems
in the performance number, we’ve adding a key to an internal database that’s driven
off this sysctl.

Comments?

Warner

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