Re: [RFC patch V2 4/7] softirq: Use hotplug thread infrastructure

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Fri, Jun 15, 2012 at 02:13:22PM -0000, Thomas Gleixner wrote:
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

And I got the following build errors on !SMP:

kernel/built-in.o: In function `run_ksoftirqd':
/usr/local/autobench/var/tmp/build/kernel/softirq.c:739: undefined reference to `.smpboot_thread_check_parking'
kernel/built-in.o: In function `watchdog':
/usr/local/autobench/var/tmp/build/kernel/watchdog.c:383: undefined reference to `.smpboot_thread_check_parking'
kernel/built-in.o: In function `spawn_ksoftirqd':
/usr/local/autobench/var/tmp/build/kernel/softirq.c:853: undefined reference to `.smpboot_register_percpu_thread'
kernel/built-in.o: In function `lockup_detector_init':
/usr/local/autobench/var/tmp/build/kernel/watchdog.c:514: undefined reference to `.smpboot_register_percpu_thread'

My guess is that !SMP needs empty definitions of these functions.

							Thanx, Paul

> ---
>  kernel/softirq.c |   84 +++++++++++--------------------------------------------
>  1 file changed, 18 insertions(+), 66 deletions(-)
> 
> Index: tip/kernel/softirq.c
> ===================================================================
> --- tip.orig/kernel/softirq.c
> +++ tip/kernel/softirq.c
> @@ -23,6 +23,7 @@
>  #include <linux/rcupdate.h>
>  #include <linux/ftrace.h>
>  #include <linux/smp.h>
> +#include <linux/smpboot.h>
>  #include <linux/tick.h>
> 
>  #define CREATE_TRACE_POINTS
> @@ -733,24 +734,17 @@ void __init softirq_init(void)
>  	open_softirq(HI_SOFTIRQ, tasklet_hi_action);
>  }
> 
> -static int run_ksoftirqd(void * __bind_cpu)
> +static int run_ksoftirqd(void *td)
>  {
> -	set_current_state(TASK_INTERRUPTIBLE);
> -
> -	while (!kthread_should_stop()) {
> +	while (!smpboot_thread_check_parking(td)) {
> +		set_current_state(TASK_INTERRUPTIBLE);
>  		preempt_disable();
> -		if (!local_softirq_pending()) {
> +		if (!local_softirq_pending())
>  			schedule_preempt_disabled();
> -		}
> 
>  		__set_current_state(TASK_RUNNING);
> 
>  		while (local_softirq_pending()) {
> -			/* Preempt disable stops cpu going offline.
> -			   If already offline, we'll be on wrong CPU:
> -			   don't process */
> -			if (cpu_is_offline((long)__bind_cpu))
> -				goto wait_to_die;
>  			local_irq_disable();
>  			if (local_softirq_pending())
>  				__do_softirq();
> @@ -758,23 +752,10 @@ static int run_ksoftirqd(void * __bind_c
>  			sched_preempt_enable_no_resched();
>  			cond_resched();
>  			preempt_disable();
> -			rcu_note_context_switch((long)__bind_cpu);
> +			rcu_note_context_switch(smp_processor_id());
>  		}
>  		preempt_enable();
> -		set_current_state(TASK_INTERRUPTIBLE);
>  	}
> -	__set_current_state(TASK_RUNNING);
> -	return 0;
> -
> -wait_to_die:
> -	preempt_enable();
> -	/* Wait for kthread_stop */
> -	set_current_state(TASK_INTERRUPTIBLE);
> -	while (!kthread_should_stop()) {
> -		schedule();
> -		set_current_state(TASK_INTERRUPTIBLE);
> -	}
> -	__set_current_state(TASK_RUNNING);
>  	return 0;
>  }
> 
> @@ -841,50 +822,17 @@ static int __cpuinit cpu_callback(struct
>  				  unsigned long action,
>  				  void *hcpu)
>  {
> -	int hotcpu = (unsigned long)hcpu;
> -	struct task_struct *p;
> -
>  	switch (action) {
> -	case CPU_UP_PREPARE:
> -	case CPU_UP_PREPARE_FROZEN:
> -		p = kthread_create_on_node(run_ksoftirqd,
> -					   hcpu,
> -					   cpu_to_node(hotcpu),
> -					   "ksoftirqd/%d", hotcpu);
> -		if (IS_ERR(p)) {
> -			printk("ksoftirqd for %i failed\n", hotcpu);
> -			return notifier_from_errno(PTR_ERR(p));
> -		}
> -		kthread_bind(p, hotcpu);
> -  		per_cpu(ksoftirqd, hotcpu) = p;
> - 		break;
> -	case CPU_ONLINE:
> -	case CPU_ONLINE_FROZEN:
> -		wake_up_process(per_cpu(ksoftirqd, hotcpu));
> -		break;
>  #ifdef CONFIG_HOTPLUG_CPU
> -	case CPU_UP_CANCELED:
> -	case CPU_UP_CANCELED_FROZEN:
> -		if (!per_cpu(ksoftirqd, hotcpu))
> -			break;
> -		/* Unbind so it can run.  Fall thru. */
> -		kthread_bind(per_cpu(ksoftirqd, hotcpu),
> -			     cpumask_any(cpu_online_mask));
>  	case CPU_DEAD:
>  	case CPU_DEAD_FROZEN: {
> -		static const struct sched_param param = {
> -			.sched_priority = MAX_RT_PRIO-1
> -		};
> -
> -		p = per_cpu(ksoftirqd, hotcpu);
> -		per_cpu(ksoftirqd, hotcpu) = NULL;
> -		sched_setscheduler_nocheck(p, SCHED_FIFO, &param);
> -		kthread_stop(p);
> +		int hotcpu = (unsigned long)hcpu;
> +
>  		takeover_tasklets(hotcpu);
>  		break;
>  	}
>  #endif /* CONFIG_HOTPLUG_CPU */
> - 	}
> +	}
>  	return NOTIFY_OK;
>  }
> 
> @@ -892,14 +840,18 @@ static struct notifier_block __cpuinitda
>  	.notifier_call = cpu_callback
>  };
> 
> +static struct smp_hotplug_thread softirq_threads = {
> +	.store		= &ksoftirqd,
> +	.thread_fn	= run_ksoftirqd,
> +	.thread_comm	= "ksoftirqd/%u",
> +};
> +
>  static __init int spawn_ksoftirqd(void)
>  {
> -	void *cpu = (void *)(long)smp_processor_id();
> -	int err = cpu_callback(&cpu_nfb, CPU_UP_PREPARE, cpu);
> -
> -	BUG_ON(err != NOTIFY_OK);
> -	cpu_callback(&cpu_nfb, CPU_ONLINE, cpu);
>  	register_cpu_notifier(&cpu_nfb);
> +
> +	BUG_ON(smpboot_register_percpu_thread(&softirq_threads));
> +
>  	return 0;
>  }
>  early_initcall(spawn_ksoftirqd);
> 
>