1. A system, comprising: a plurality of CPUs; a memory coupled to said CPUs, said memory including a computer readable storage medium tangibly embodying at least one program of instructions executable by said CPUs to perform hierarchical read-copy update (RCU) grace period detection operations that accommodate CPU-hotplugging while avoiding degradation of real-time latency due to task list migration, said operations comprising: providing a combining tree of two or more levels to track quiescent states, said levels including a leaf level and a root level; determining whether a condition warrants adjusting hierarchical RCU grace period detection to account for CPUs that have gone offline due to hotplugging operations; said condition being met if (1) a leaf level bitmask of said combining tree is fully cleared to indicate an offline state in which all of its assigned CPUs are offline, and (2) there are no tasks on a task list associated with said fully cleared bitmask; if said condition is met, then propagating said offline state of said leaf level bitmask up said combining tree to a root level bitmask to thereby establish an adjusted grace period detection state in which said fully cleared bitmask's assigned CPUs are ignored for purposes of grace period detection; if said condition is partially met due to said leaf level bitmask being fully cleared to indicate an offline state in which all of its assigned CPUs are offline, but there are tasks remaining on said task list associated with said fully cleared bitmask, then (3) refraining from propagating said offline state up said combining tree to said root level bitmask and (4) maintaining said associated task list at said leaf level without migration to said root level, thereby ensuring that said tasks remaining on said task list will continue to have effect for purposes of grace period detection without having to migrate said task list; and unless said condition is fully met, continuing existing grace period detection without propagating said offline state.