  With the shrinking of technology nodes and the use of parallel processor
clusters in hostile and critical environments, such as space, run-time faults
caused by radiation are a serious cross-cutting concern, also impacting
architectural design. This paper introduces an architectural approach to
run-time configurable soft-error tolerance at the core level, augmenting a
six-core open-source RISC-V cluster with a novel On-Demand Redundancy Grouping
(ODRG) scheme. ODRG allows the cluster to operate either as two fault-tolerant
cores, or six individual cores for high-performance, with limited overhead to
switch between these modes during run-time. The ODRG unit adds less than 11% of
a core's area for a three-core group, or a total of 1% of the cluster area, and
shows negligible timing increase, which compares favorably to a commercial
state-of-the-art implementation, and is 2.5$\times$ faster in fault recovery
re-synchronization. Furthermore, when redundancy is not necessary, the ODRG
approach allows the redundant cores to be used for independent computation,
allowing up to 2.96$\times$ increase in performance for selected applications.
