Nothing busy looped getpid. A library used getpid on each entry to check if the process had forked. The library was called many times.
getpid was a fairly common way to check for fork prior to mechanisms like madvise WIPEONFORK (Linux 4.14, released in 2017). systemd existed prior to 2017 and likely needed to support older kernels past 4.14's initial release. And the getpid cache was something glibc had prior to April 2017, likely to support this kind of use case. It was even documented behavior[1].
So, anyway, what systemd was doing wasn't totally stupid. It became a lot more expensive when glibc broke it on them and then it needed to be improved. But before that it wasn't as objectionable as people in this thread are suggesting.
(Also: pthread_atfork requires linking pthreads, which is or at least historically was seen as a significant burden on applications which do not use pthreads.)
getpid was a fairly common way to check for fork prior to mechanisms like madvise WIPEONFORK (Linux 4.14, released in 2017). systemd existed prior to 2017 and likely needed to support older kernels past 4.14's initial release. And the getpid cache was something glibc had prior to April 2017, likely to support this kind of use case. It was even documented behavior[1].
So, anyway, what systemd was doing wasn't totally stupid. It became a lot more expensive when glibc broke it on them and then it needed to be improved. But before that it wasn't as objectionable as people in this thread are suggesting.
(Also: pthread_atfork requires linking pthreads, which is or at least historically was seen as a significant burden on applications which do not use pthreads.)
[1]: https://bugzilla.redhat.com/show_bug.cgi?id=1469670#c9