• Peter Zijlstra's avatar
    sched: Cleanup cpu_active madness · 5fbd036b
    Peter Zijlstra authored
    Stepan found:
    
    CPU0		CPUn
    
    _cpu_up()
      __cpu_up()
    
    		boostrap()
    		  notify_cpu_starting()
    		  set_cpu_online()
    		  while (!cpu_active())
    		    cpu_relax()
    
    <PREEMPT-out>
    
    smp_call_function(.wait=1)
      /* we find cpu_online() is true */
      arch_send_call_function_ipi_mask()
    
      /* wait-forever-more */
    
    <PREEMPT-in>
    		  local_irq_enable()
    
      cpu_notify(CPU_ONLINE)
        sched_cpu_active()
          set_cpu_active()
    
    Now the purpose of cpu_active is mostly with bringing down a cpu, where
    we mark it !active to avoid the load-balancer from moving tasks to it
    while we tear down the cpu. This is required because we only update the
    sched_domain tree after we brought the cpu-down. And this is needed so
    that some tasks can still run while we bring it down, we just don't want
    new tasks to appear.
    
    On cpu-up however the sched_domain tree doesn't yet include the new cpu,
    so its invisible to the load-balancer, regardless of the active state.
    So instead of setting the active state after we boot the new cpu (and
    consequently having to wait for it before enabling interrupts) set the
    cpu active before we set it online and avoid the whole mess.
    Reported-by: default avatarStepan Moskovchenko <stepanm@codeaurora.org>
    Signed-off-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
    Acked-by: default avatarThomas Gleixner <tglx@linutronix.de>
    Link: http://lkml.kernel.org/r/1323965362.18942.71.camel@twinsSigned-off-by: default avatarIngo Molnar <mingo@elte.hu>
    5fbd036b
smp.c 27.4 KB