Re: [PATCH] sched: Allow per-cpu kernel threads to run on online && !active

[Peter Zijlstra]

On Thu, Mar 03, 2016 at 12:43:53PM +0100, Thomas Gleixner wrote:

> Subject: cpu/hotplug: Handle cpu active as last state
> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Date: Thu, 03 Mar 2016 12:33:37 +0100
> 
> We want to make sure that everything is initialized before we allow scheduling
> of arbitrary work on a upcoming CPU and we don't want to have no more random
> work on it when we shut it down.
> 
> Now that the scheduler handles this nicely via the cpu_active_mask we can put
> set_cpu_active() as last action when a cpu is brought up and as first action
> when it goes down.
> 
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

Nice!

Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

> ---
>  include/linux/cpuhotplug.h |    1 +
>  kernel/cpu.c               |   19 +++++++++++++++++--
>  kernel/sched/core.c        |   18 ------------------
>  3 files changed, 18 insertions(+), 20 deletions(-)
> 
> +++ b/kernel/sched/core.c
> @@ -5726,23 +5726,6 @@ static int sched_cpu_active(struct notif
>  	case CPU_STARTING:
>  		set_cpu_rq_start_time();

If we stick that in notify_starting() we can kill all of this.

>  		return NOTIFY_OK;
> -
> -	case CPU_DOWN_FAILED:
> -		set_cpu_active(cpu, true);
> -		return NOTIFY_OK;
> -
> -	default:
> -		return NOTIFY_DONE;
> -	}
> -}
> -
> -static int sched_cpu_inactive(struct notifier_block *nfb,
> -					unsigned long action, void *hcpu)
> -{
> -	switch (action & ~CPU_TASKS_FROZEN) {
> -	case CPU_DOWN_PREPARE:
> -		set_cpu_active((long)hcpu, false);
> -		return NOTIFY_OK;
>  	default:
>  		return NOTIFY_DONE;
>  	}
> @@ -5761,7 +5744,6 @@ static int __init migration_init(void)
>  
>  	/* Register cpu active notifiers */
>  	cpu_notifier(sched_cpu_active, CPU_PRI_SCHED_ACTIVE);
> -	cpu_notifier(sched_cpu_inactive, CPU_PRI_SCHED_INACTIVE);
>  
>  	return 0;
>  }