file: /proc/sys/net/core/netdev_tstamp_prequeue
variable: net.core.netdev_tstamp_prequeue
Official reference

If set to 0, RX packet timestamps can be sampled after RPS processing, when the target CPU processes packets. It might give some delay on timestamps, but permit to distribute the load on several cpus.

If set to 1 (default), timestamps are sampled as soon as possible, before queueing.