[$] The 7.0 scheduler regression that wasn't

One of the more significant changes in the 7.0 kernel release is to use the lazy-preemption mode by default in the CPU scheduler. The scheduler developers have wanted to reduce the number of preemption modes for years, and lazy preemption looks like a step toward that goal. But then there came this report from Salvatore Dipietro that lazy preemption caused a 50% performance regression on a PostgreSQL benchmark. Investigation showed that the situation is not actually so grave, but the episode highlights just how sensitive some workloads can be to configuration changes; there may be surprises in store for other users as well.