Systems with multiple CPUs are reasonably rare in the Linux world but a lot of work has already gone into making Linux an SMP (Symmetric Multi-Processing) operating system. That is, one that is capable of evenly balancing work between the CPUs in the system. Nowhere is this balancing of work more apparent than in the scheduler.
In a multiprocessor system hopefully all of the processors are busily running processes. Each will run the scheduler separately as its current process exhausts its time-slice or has to wait for a system resource. The first thing to notice about an SMP system is that there is not just one idle process in the system. In a single processor system the idle process is the first task in the task vector, in an SMP system there is one idle process per CPU as you could have more than one idle CPU. Additionally there is one current process per CPU, so SMP systems must keep track of the current and idle processes for each processor.
In an SMP system each processes task_struct contains the number of the processor that it is currently running on (processor ) and its processor number of the last processor that it ran on (last_processor ). There is no reason why a process should not run on a different CPU each time it is selected to run but Linux can restrict a process to one or more processors in the system using the processor_mask . If bit N is set, then this process can run on processor N. When the scheduler is choosing a new process to run it will not consider one that does have the appropriate bit set for the current processor's number in its processor_mask . The scheduler also gives a slight advantage to a process that last run on the current processor because there is often a performance overhead when moving a process to a different processor.