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SubjectRe: [PATCH 3/5] x86, acpi, cpu-hotplug: Introduce apicid_to_cpuid[] array to store persistent cpuid <-> apicid mapping.
Hello,

On Tue, Jul 07, 2015 at 05:30:23PM +0800, Tang Chen wrote:
> From: Gu Zheng <guz.fnst@cn.fujitsu.com>

It would be a good idea to briefly describe what the overall goal is
and why we want that.

> In this patch, we introduce a new static array named apicid_to_cpuid[],
> which is large enough to store info for all possible cpus.
>
> And then, we modify the cpuid calculation. In generic_processor_info(),
> it simply finds the next unused cpuid. And it is also why the cpuid <-> nodeid
> mapping changes with node hotplug.
>
> After this patch, we find the next unused cpuid, map it to an apicid,
> and store the mapping in apicid_to_cpuid[], so that cpuid <-> apicid
> mapping will be persistent.
>
> And finally we will use this array to make cpuid <-> nodeid persistent.
>
> cpuid <-> apicid mapping is established at local apic registeration time.
> But non-present or disabled cpus are ignored.
>
> In this patch, we establish all possible cpuid <-> apicid mapping when
> registering local apic.
>
>
> Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
> Signed-off-by: Tang Chen <tangchen@cn.fujitsu.com>
> ---
...
> diff --git a/arch/x86/kernel/acpi/boot.c b/arch/x86/kernel/acpi/boot.c
> index e49ee24..bcc85b2 100644
> --- a/arch/x86/kernel/acpi/boot.c
> +++ b/arch/x86/kernel/acpi/boot.c
> @@ -174,15 +174,13 @@ static int acpi_register_lapic(int id, u8 enabled)
> return -EINVAL;
> }
>
> - if (!enabled) {
> + if (!enabled)
> ++disabled_cpus;
> - return -EINVAL;
> - }
>
> if (boot_cpu_physical_apicid != -1U)
> ver = apic_version[boot_cpu_physical_apicid];
>
> - return generic_processor_info(id, ver);
> + return __generic_processor_info(id, ver, enabled);
> }
>
> static int __init
> diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
> index a9c9830..c744ffb 100644
> --- a/arch/x86/kernel/apic/apic.c
> +++ b/arch/x86/kernel/apic/apic.c
> @@ -1977,7 +1977,38 @@ void disconnect_bsp_APIC(int virt_wire_setup)
> apic_write(APIC_LVT1, value);
> }
>
> -static int __generic_processor_info(int apicid, int version, bool enabled)
> +/*
> + * Logic cpu number(cpuid) to local APIC id persistent mappings.

Logical

Also, isn't it the other way around?

> + * Do not clear the mapping even if cpu is hot-removed.
> + */
> +static int apicid_to_cpuid[] = {
> + [0 ... NR_CPUS - 1] = -1,
> +};
> +
> +/*
> + * Internal cpu id bits, set the bit once cpu present, and never clear it.
> + */
> +static cpumask_t cpuid_mask = CPU_MASK_NONE;
> +
> +static int get_cpuid(int apicid)
> +{
> + int free_id, i;
> +
> + free_id = cpumask_next_zero(-1, &cpuid_mask);
> + if (free_id >= nr_cpu_ids)
> + return -1;
> +
> + for (i = 0; i < free_id; i++)
> + if (apicid_to_cpuid[i] == apicid)
> + return i;
> +
> + apicid_to_cpuid[free_id] = apicid;
> + cpumask_set_cpu(free_id, &cpuid_mask);
> +
> + return free_id;

Why can't this function simply test whether apicid_to_cpuid[] is -1 or
not? Also, why does it need cpuid_mask? Isn't it just giving out cpu
id numbers sequentially?

> +}
> +
> +int __generic_processor_info(int apicid, int version, bool enabled)
> {
> int cpu, max = nr_cpu_ids;
> bool boot_cpu_detected = physid_isset(boot_cpu_physical_apicid,
> @@ -2058,8 +2089,18 @@ static int __generic_processor_info(int apicid, int version, bool enabled)
> * for BSP.
> */
> cpu = 0;
> - } else
> - cpu = cpumask_next_zero(-1, cpu_present_mask);
> + } else {
> + cpu = get_cpuid(apicid);
> + if (cpu < 0) {
> + int thiscpu = max + disabled_cpus;
> +
> + pr_warning(" Processor %d/0x%x ignored.\n",
> + thiscpu, apicid);

Given that the only failing condition is there are more possible cpus
than nr_cpu_ids, it might make more sense to warn this once in
get_cpuid().

Also, wouldn't it make more sense / safer to allocate all online cpus
first and then go through possible cpus?

Thanks.

--
tejun


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