Re: [PATCH] cpu: Speedup disable_nonboot_cpus()

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 3 May 2013 17:35:44 -0500 Russ Anderson <rja@sgi.com> wrote:

> The routine disable_nonboot_cpus() shuts down cpus sequentially
> using for_each_online_cpu(cpu) to call cpu_down() one cpu at
> a time.  cpu_down() calls __stop_machine() which stops all
> the cpus while it disables one.  Then it re-enables the remaining
> cpus, only to do it all over again for the next cpu.  The 
> result is that it takes 16 minutes on a 1024 cpu system to 
> disable 1023 cpus.
> 
> This patch changes disable_nonboot_cpus() to pass a bitmask
> of cpus to cpu_down() and modifies cpu_down() to only call
> __stop_machine() once.
> 
> On a 1024 cpu system this reduces the time it takes to disable
> all but one cpu from 16 minutes down to 4 minutes.

That's still a helluva long time.  What's the kernel *doing* for
all that time?

> --- linux.orig/kernel/cpu.c	2013-05-03 09:56:31.145508321 -0500
> +++ linux/kernel/cpu.c	2013-05-03 17:01:20.652959400 -0500
>
> ...
>
> @@ -255,21 +255,21 @@ static int __ref take_cpu_down(void *_pa
>  	if (err < 0)
>  		return err;
>  
> -	cpu_notify(CPU_DYING | param->mod, param->hcpu);
> +	cpu_notify(CPU_DYING | param->mod, hcpu);
>  	/* Park the stopper thread */
>  	kthread_park(current);
>  	return 0;
>  }
>  
>  /* Requires cpu_add_remove_lock to be held */
> -static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
> +static int __ref _cpu_down(const cpumask_t *cpus_to_offline, int tasks_frozen)

_cpu_down() is now misnamed - it downs multiple CPUs.

>  {
> -	int err, nr_calls = 0;
> +	int err = 0, cpu = 0, nr_calls = 0;
>  	void *hcpu = (void *)(long)cpu;
> +	cpumask_var_t cpus_offlined;
>  	unsigned long mod = tasks_frozen ? CPU_TASKS_FROZEN : 0;
>  	struct take_cpu_down_param tcd_param = {
>  		.mod = mod,
> -		.hcpu = hcpu,
>  	};
>  
>  	if (num_online_cpus() == 1)
> @@ -278,46 +278,67 @@ static int __ref _cpu_down(unsigned int
>  	if (!cpu_online(cpu))
>  		return -EINVAL;
>  
> +	if (!alloc_cpumask_var(&cpus_offlined, GFP_KERNEL))
> +		return -ENOMEM;
> +
>  	cpu_hotplug_begin();
> +	cpumask_copy(cpus_offlined, cpus_to_offline);
>  
> -	err = __cpu_notify(CPU_DOWN_PREPARE | mod, hcpu, -1, &nr_calls);
> -	if (err) {
> -		nr_calls--;
> -		__cpu_notify(CPU_DOWN_FAILED | mod, hcpu, nr_calls, NULL);
> -		printk("%s: attempt to take down CPU %u failed\n",
> +	for_each_cpu_mask(cpu, *cpus_to_offline) {
> +		if (!cpu_online(cpu))
> +			continue;
> +		hcpu = (void *)(long)cpu;
> +		err = __cpu_notify(CPU_DOWN_PREPARE | mod, hcpu, -1, &nr_calls);
> +		if (err) {
> +			nr_calls--;
> +			__cpu_notify(CPU_DOWN_FAILED | mod, hcpu, nr_calls, NULL);
> +			pr_err("%s: attempt to take down CPU %u failed\n",
>  				__func__, cpu);
> -		goto out_release;
> +			goto out_release;
> +		}
> +		smpboot_park_threads(cpu);
>  	}
> -	smpboot_park_threads(cpu);
>  
> -	err = __stop_machine(take_cpu_down, &tcd_param, cpumask_of(cpu));
> +	err = __stop_machine(take_cpu_down, &tcd_param, cpus_to_offline);
>  	if (err) {
>  		/* CPU didn't die: tell everyone.  Can't complain. */

This comment is now inaccurate.  "One or more of the CPUs didn't die"?.

I'm not sure what "Can't complain" means.  Perhaps expand on this while
you're there?

> -		smpboot_unpark_threads(cpu);
> -		cpu_notify_nofail(CPU_DOWN_FAILED | mod, hcpu);
> +		for_each_cpu_mask(cpu, *cpus_to_offline) {
> +			hcpu = (void *)(long)cpu;
> +			smpboot_unpark_threads(cpu);
> +			cpu_notify_nofail(CPU_DOWN_FAILED | mod, hcpu);

Is this accurate?  What happens if we asked stop_machine() to down 100
CPUs but it failed at CPU #50?  We now tell listeners that we failed to
down all 100 CPUs?  That's not true.

> +		}
>  		goto out_release;
>  	}
> -	BUG_ON(cpu_online(cpu));
>  
>  	/*
>  	 * The migration_call() CPU_DYING callback will have removed all
>  	 * runnable tasks from the cpu, there's only the idle task left now
>  	 * that the migration thread is done doing the stop_machine thing.
> -	 *
> -	 * Wait for the stop thread to go away.
>  	 */
> -	while (!idle_cpu(cpu))
> -		cpu_relax();
> -
> -	/* This actually kills the CPU. */
> -	__cpu_die(cpu);
> +	for_each_cpu_mask(cpu, *cpus_offlined) {
> +		BUG_ON(cpu_online(cpu));
>  
> -	/* CPU is completely dead: tell everyone.  Too late to complain. */
> -	cpu_notify_nofail(CPU_DEAD | mod, hcpu);
> -
> -	check_for_tasks(cpu);
> +		/*
> +		 * Wait for the stop thread to go away.
> +		 */
> +		while (!idle_cpu(cpu))
> +			cpu_relax();
> +
> +		/*
> +		 * This actually kills the CPU.
> +		 */
> +		__cpu_die(cpu);
> +
> +		/*
> +		 * CPU is completely dead: tell everyone.  Too late to complain.
> +		 */
> +		hcpu = (void *)(long)cpu;
> +		cpu_notify_nofail(CPU_DEAD | mod, hcpu);
> +		check_for_tasks(cpu);
> +	}
>  
>  out_release:
> +	free_cpumask_var(cpus_offlined);
>  	cpu_hotplug_done();

Swap the above two lines and we reduced the locked region by an
unmeasurable amount!

>  	if (!err)
>  		cpu_notify_nofail(CPU_POST_DEAD | mod, hcpu);
>
> ...
>