A data processing apparatus and method are provided for handling execution of instructions within a data processing apparatus having a plurality of processing units. Each processing unit is operable to execute a sequence of instructions so as to perform associated operations, and at least a subset of the processing units form a cluster. Instruction forwarding logic is provided which for at least one instruction executed by at least one of the processing units in the cluster causes that instruction to be executed by each of the other processing units in the cluster, for example by causing that instruction to be inserted into the sequences of instructions executed by each of the other processing units in the cluster. Such a mechanism provides a particularly efficient technique to cause each of the processing units in the cluster to perform an identical operation which can be useful in a variety of situations, for example in an SMP system where coherency of data is required and accordingly maintenance operations need to be performed periodically within each processing unit to ensure such coherency.