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Date:	Fri, 8 Aug 2014 16:25:02 +0530
From:	Gautham R Shenoy <ego@...ux.vnet.ibm.com>
To:	Lan Tianyu <tianyu.lan@...el.com>
Cc:	rjw@...ysocki.net, len.brown@...el.com, pavel@....cz,
	peterz@...radead.org, toshi.kani@...com, mingo@...nel.org,
	akpm@...ux-foundation.org, todd.e.brandt@...ux.intel.com,
	fabf@...net.be, srivatsa.bhat@...ux.vnet.ibm.com,
	ego@...ux.vnet.ibm.com, rafael.j.wysocki@...el.com,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [RFC PATCH] PM/CPU: Parallel enabling nonboot cpus with resume
 devices

Hi Lan,

On Wed, Jul 23, 2014 at 05:11:34PM +0800, Lan Tianyu wrote:
> In the current world, all nonboot cpus are enabled serially during system
> resume. System resume sequence is that boot cpu enables nonboot cpu one by
> one and then resume devices. Before resuming devices, there are few tasks
> assigned to nonboot cpus after they are brought up. This waste cpu usage.
> 
> To accelerate S3, this patches adds a new kernel configure
> PM_PARALLEL_CPU_UP_FOR_SUSPEND to allow boot cpu to go forward to resume
> devices after bringing up one nonboot cpu. The nonboot cpu will be in charge
> of bringing up other cpus. This makes enabling cpu2~x parallel with resuming
> devices. From the test result on 4 logical core laptop, the time of resume
> device almost wasn't affected by enabling nonboot cpus lately while the start
> point is almost 30ms earlier than before.
> 
> Signed-off-by: Lan Tianyu <tianyu.lan@...el.com>
> ---
>  kernel/cpu.c         | 82 ++++++++++++++++++++++++++++++++++++++++++++++------
>  kernel/power/Kconfig | 13 +++++++++
>  2 files changed, 86 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index a343bde..d4c1353 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -551,9 +551,27 @@ void __weak arch_enable_nonboot_cpus_end(void)
>  {
>  }
> 
> +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;
> +}
> +

[..snip..]
> 
> +
> +void __ref enable_nonboot_cpus(void)
> +{
> +	struct task_struct *tsk;
> +	int cpu;
> +
> +	/* Allow everyone to use the CPU hotplug again */
> +	cpu_maps_update_begin();
> +	cpu_hotplug_disabled = 0;
> +	if (cpumask_empty(frozen_cpus))
> +		goto out;
> +
> +	arch_enable_nonboot_cpus_begin();
> +
> +	cpu = cpumask_first(frozen_cpus);
> +	cpumask_clear_cpu(cpu, frozen_cpus);
> +
> +	_cpu_up_with_trace(cpu);

We should be handling the error returned by _cpu_up_with_trace() 
in case 'cpu' fails to come online. Unless it is something that I am
not aware of it doesn't make much sense to create a kthread on a cpu
that we know has failed to come online.


> +
> +	if (cpumask_empty(frozen_cpus)) {
> +		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;
> +		}
> +
> +		kthread_unpark(tsk);
> +	}
> +out:
> +	cpu_maps_update_done();
> +}
> +#endif
> +

--
Thanks and Regards
gautham.

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