linux-kernel - Re: [RFC PATCH V2] PM/CPU: Parallel enalbing nonboot cpus with resume devices

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Date:	Wed, 17 Sep 2014 14:33:08 +0530
From:	Gautham R Shenoy <ego@...ux.vnet.ibm.com>
To:	Lan Tianyu <tianyu.lan@...el.com>
Cc:	peterz@...radead.org, mingo@...nel.org, rafael.j.wysocki@...el.com,
	srivatsa@....edu, akpm@...ux-foundation.org, laijs@...fujitsu.com,
	toshi.kani@...com, todd.e.brandt@...ux.intel.com,
	wangyun@...ux.vnet.ibm.com, ego@...ux.vnet.ibm.com, fabf@...net.be,
	linux@....linux.org.uk, oleg@...hat.com,
	srivatsa.bhat@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH V2] PM/CPU: Parallel enalbing nonboot cpus with
 resume devices

Hi Lan,

Sorry missed this repost! Couple of comments.

On Fri, Aug 22, 2014 at 04:33:40PM +0800, Lan Tianyu wrote:

[.. snip ..]
> 
> +static int _cpu_up_with_trace(int cpu)
> +{
> +	int error;
> +
> +	trace_suspend_resume(TPS("CPU_ON"), cpu, true);
> +	error = _cpu_up(cpu, 1);
> +	trace_suspend_resume(TPS("CPU_ON"), cpu, false);
> +	if (error) {
> +		pr_warn("Error taking CPU%d up: %d\n", cpu, error);
> +		return error;
> +	}
> +
> +	pr_info("CPU%d is up\n", cpu);
> +	return 0;
> +}
> +
> +static int async_enable_nonboot_cpus(void *data)
> +{
> +	int cpu;
> +
> +	cpu_maps_update_begin();
> +	arch_enable_nonboot_cpus_begin();
> +
> +	for_each_cpu(cpu, frozen_cpus) {
> +		_cpu_up_with_trace(cpu);
> +	}
> +
> +	arch_enable_nonboot_cpus_end();
> +	cpumask_clear(frozen_cpus);
> +	cpu_maps_update_done();
> +	return 0;
> +}
> +
>  void __ref enable_nonboot_cpus(void)
>  {
> +	struct task_struct *tsk;
>  	int cpu, error;
> 
>  	/* Allow everyone to use the CPU hotplug again */
> @@ -563,22 +597,34 @@ void __ref enable_nonboot_cpus(void)
> 
>  	pr_info("Enabling non-boot CPUs ...\n");
> 
> -	arch_enable_nonboot_cpus_begin();
> +	cpu = cpumask_first(frozen_cpus);
> +	cpumask_clear_cpu(cpu, frozen_cpus);
> 
> -	for_each_cpu(cpu, frozen_cpus) {
> -		trace_suspend_resume(TPS("CPU_ON"), cpu, true);
> -		error = _cpu_up(cpu, 1);
> -		trace_suspend_resume(TPS("CPU_ON"), cpu, false);
> -		if (!error) {
> -			pr_info("CPU%d is up\n", cpu);
> -			continue;
> +	error = _cpu_up_with_trace(cpu);

 If cpu fails to come up, you need to add a pr_warn() citing the
 reason why it failed to come up.


> +	if (cpumask_empty(frozen_cpus))
> +		goto out;
> +
> +	if (error) {
> +		/*
> +		 * If fail to bring up the first frozen cpus,
> +		 * enable the rest frozen cpus on the boot cpu.
> +		 */
> +		arch_enable_nonboot_cpus_begin();
> +		for_each_cpu(cpu, frozen_cpus) {
> +			_cpu_up_with_trace(cpu);
>  		}
> -		pr_warn("Error taking CPU%d up: %d\n", cpu, error);
> -	}
> +		arch_enable_nonboot_cpus_end();
> 
> -	arch_enable_nonboot_cpus_end();
> +	} else {
> +		tsk = kthread_create_on_cpu(async_enable_nonboot_cpus,
> +				NULL, cpu, "async-enable-nonboot-cpus");
> +		if (IS_ERR(tsk)) {
> +			pr_err("Failed to create async enable nonboot cpus thread.\n");
> +			goto out;

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ This is not good. If you fail to
create a kthread on the first non-boot cpu, that means none of the
other non-boot cpus will be brought up. 

Hence you might want to consider reordering the code in such a manner
that if the first non-boot cpu fails to come up or if you fail to
create the kthread task, then the boot cpu will boot the remaining non
boot cpus.

--
Thanks and Regards
gautham.

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