linux-kernel - Re: [PATCH v2 5/7] x86, acpi, cpu-hotplug: Introduce apicid_to_cpuid[] array to store persistent cpuid <-> apicid mapping.

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Date:	Thu, 10 Sep 2015 15:55:32 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Tang Chen <tangchen@...fujitsu.com>
Cc:	jiang.liu@...ux.intel.com, mika.j.penttila@...il.com,
	mingo@...hat.com, akpm@...ux-foundation.org, rjw@...ysocki.net,
	hpa@...or.com, yasu.isimatu@...il.com,
	isimatu.yasuaki@...fujitsu.com, kamezawa.hiroyu@...fujitsu.com,
	izumi.taku@...fujitsu.com, gongzhaogang@...pur.com,
	qiaonuohan@...fujitsu.com, x86@...nel.org,
	linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Gu Zheng <guz.fnst@...fujitsu.com>
Subject: Re: [PATCH v2 5/7] x86, acpi, cpu-hotplug: Introduce
 apicid_to_cpuid[] array to store persistent cpuid <-> apicid mapping.

Hello,

So, overall, I think this is the right way to go although I have no
idea whether the acpi part is okay.

> +/*
> + * Current allocated max logical CPU ID plus 1.
> + * All allocated CPU ID should be in [0, max_logical_cpuid),
> + * so the maximum of max_logical_cpuid is nr_cpu_ids.
> + *
> + * NOTE: Reserve 0 for BSP.
> + */
> +static int max_logical_cpuid = 1;

Rename it to nr_logical_cpuids and just mention that it's allocated
contiguously?

> +static int cpuid_to_apicid[] = {
> +	[0 ... NR_CPUS - 1] = -1,
> +};

And maybe mention how the two variables are synchronized?

> +static int allocate_logical_cpuid(int apicid)
> +{
> +	int i;
> +
> +	/*
> +	 * cpuid <-> apicid mapping is persistent, so when a cpu is up,
> +	 * check if the kernel has allocated a cpuid for it.
> +	 */
> +	for (i = 0; i < max_logical_cpuid; i++) {
> +		if (cpuid_to_apicid[i] == apicid)
> +			return i;
> +	}
> +
> +	/* Allocate a new cpuid. */
> +	if (max_logical_cpuid >= nr_cpu_ids) {
> +		WARN_ONCE(1, "Only %d processors supported."
> +			     "Processor %d/0x%x and the rest are ignored.\n",
> +			     nr_cpu_ids - 1, max_logical_cpuid, apicid);
> +		return -1;
> +	}

So, the original code didn't have this failure mode, why is this
different for the new code?

Thanks.

-- 
tejun
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