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:	Sat, 26 Sep 2015 17:52:09 +0800
From:	Tang Chen <tangchen@...fujitsu.com>
To:	Tejun Heo <tj@...nel.org>
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>, <tangchen@...fujitsu.com>
Subject: Re: [PATCH v2 5/7] x86, acpi, cpu-hotplug: Introduce apicid_to_cpuid[]
 array to store persistent cpuid <-> apicid mapping.

Hi tj,

On 09/11/2015 03:55 AM, Tejun Heo wrote:
> Hello,
>
> So, overall, I think this is the right way to go although I have no
> idea whether the acpi part is okay.

Thank you very much for reviewing. :)

>
>> +/*
>> + * 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?

OK.

>
>> +static int cpuid_to_apicid[] = {
>> +	[0 ... NR_CPUS - 1] = -1,
>> +};
> And maybe mention how the two variables are synchronized?

User should call allocate_logical_cpuid() to get a new logical cpuid.
This allocator will ensure the synchronization.

Will mention it in the comment.

>
>> +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?

It is not different. Since max_logical_cpuid is new, this is ensure it 
won't
go beyond NR_CPUS.

Thanks.

>
> Thanks.
>

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