The problem/use-case that the feature addresses
Having both the main redis_server thread and the i/o threads share the same CPU cores can degrate performance when enabling i/o threads on certain OS scheduling conditions.
Description of the feature
Due to problematic OS scheduling, we can sometime encounter situations where both the i/o threads and main redis_server thread use the same core, even though they all have a wider core mask (say cores 0-3). In even more limited environments (say server_cpulist is set to 0,1) it makes no sense, and even counteracts the purpose of moving i/o workloads to different threads, to have these threads compete for the same compute resources. Thus I suggest having another variable iothreads_cpulist, allowing us to put these threads on separate cores.
Alternatives you've considered
None are available, accept for manually identifying and setting the io-threads affinity to use different cores