How to run multiple process, sequentially in gem5 se mode?

Question

I followed gem5-tutorial to build a test config. It executes hello world, on se mode. But, now I want to run multiple processes, one by one. How to do that? So far i tried this

processes = []
processes.append([bzip2_benchmark, bzip2_input])
processes.append([mcf_benchmark, mcf_input])
processes.append([hmmer_benchmark, '--fixed=0', '--mean=325', '--num=45000', '--sd=200', '--seed=0', hmmer_input])
processes.append([sjeng_benchmark, sjeng_input])
processes.append([lbm_benchmark, 20, 'reference.dat', 0, 1, benchmark_dir+'470.lbm/data/100_100_130_cf_a.of'])
for p in processes:
    process = Process()
    process.cmd = p
    system.cpu.workload = process
    system.cpu.createThreads()

    root = Root(full_system=False, system=system)
    m5.instantiate()

    print("Beginning simulation!")
    exit_event = m5.simulate()

    print('Exiting @ tick {} because {}'
    .format(m5.curTick(), exit_event.getCause()))

Assume that all imports are correct and, system is instantiated correctly. The above code gives "fatal: Attempt to allocate multiple instances of Root.", after running the first process. I understood why this happens, but I want to know how to run these benchmark programs one by one.