Not exactly. It depends on the syscall. Syscalls that are known to be slow are treated the way you're describing, but syscalls that are often fast are treated optimistically in a way that allows the work queue to be stolen and spawned onto a new thread only if the syscall doesn't return quickly.

I don't know which category epoll fits into, but it seems like it should be treated optimistically, since epoll is used for non-blocking I/O.

https://utcc.utoronto.ca/~cks/space/blog/programming/GoSched...