Re: [PATCH v4] x86,cpu-hotplug: assign same CPU number to readded CPU

[Yasuaki Ishimatsu]

Ping.

(2014/07/23 12:22), Yasuaki Ishimatsu wrote:
> llc_shared_map is not cleared even if CPU is offline or hot removed.
> So when hot-plugging CPU and assigning new CPU number to hot-added CPU,
> the mask has wrong value. The mask is used by CSF schduler to create
> sched_domain. So it breaks CFS scheduler.
>
> Here is a example on my system.
> My system has 4 sockets and each socket has 15 cores and HT is enabled.
> In this case, each core of sockes is numbered as follows:
>
>           | CPU#
> Socket#0 | 0-14 , 60-74
> Socket#1 | 15-29, 75-89
> Socket#2 | 30-44, 90-104
> Socket#3 | 45-59, 105-119
>
> Then llc_shared_mask of CPU#30 has 0x3fff80000001fffc0000000.
> It means that last level cache of Socket#2 is shared with
> CPU#30-44 and 90-104.
>
> When hot-removing socket#2 and #3, each core of sockets is numbered
> as follows:
>
>           | CPU#
> Socket#0 | 0-14 , 60-74
> Socket#1 | 15-29, 75-89
>
> But llc_shared_mask is not cleared. So llc_shared_mask of CPU#30 remains
> having 0x3fff80000001fffc0000000.
>
> After that, when hot-adding socket#2 and #3, each core of sockets is
> numbered as follows:
>
>           | CPU#
> Socket#0 | 0-14 , 60-74
> Socket#1 | 15-29, 75-89
> Socket#2 | 30-59
> Socket#3 | 90-119
>
> Then llc_shared_mask of CPU#30 becomes 0x3fff8000fffffffc0000000.
> It means that last level cache of Socket#2 is shared with CPU#30-59
> and 90-104. So the mask has wrong value.
>
> At first, I cleared hot-removed CPU number's bit from llc_shared_map
> when hot removing CPU. But Borislav suggested that the problem will
> disappear if readded CPU is assigned same CPU number. And llc_shared_map
> must not be changed.
>
> So the patch assigns same CPU number to readded CPU by linking CPU
> number to APIC ID. And by the patch, the problem disappers.
>
> Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@xxxxxxxxxxxxxx>
> Suggested-by: Borislav Petkov <bp@xxxxxxxxx>
> Reviewed-by: Toshi Kani <toshi.kani@xxxxxx>
> ---
> v2: change cpuid to cpunum
> v3: fix Borislav's email address of Suggested-by
>      fix typo (ACPI ID to APIC ID)
> v4: change cpu_used_xxx to cpu_number_xxx
> ---
>   arch/x86/kernel/apic/apic.c | 33 ++++++++++++++++++++++++++++++++-
>   1 file changed, 32 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
> index ad28db7..5cecc3b 100644
> --- a/arch/x86/kernel/apic/apic.c
> +++ b/arch/x86/kernel/apic/apic.c
> @@ -220,6 +220,23 @@ static void apic_pm_activate(void);
>   static unsigned long apic_phys;
>
>   /*
> + * Bind APIC ID to Logical CPU number
> + * Logical CPU number to APIC ID does not change by this array
> + * even if CPU is hotplugged. So don't clear the array even if
> + * CPU is hot-removed
> + */
> +static int apicid_to_cpunum[MAX_LOCAL_APIC] = {
> +    [0 ... MAX_LOCAL_APIC-1] = -1,
> +};
> +
> +/*
> + * Represent Logical CPU number bound to APIC ID
> + * Don't clear a bit even if CPU is hot-removed
> + */
> +static DECLARE_BITMAP(cpu_number_bits, CONFIG_NR_CPUS);
> +static struct cpumask *const cpu_number_mask = to_cpumask(cpu_number_bits);
> +
> +/*
>    * Get the LAPIC version
>    */
>   static inline int lapic_get_version(void)
> @@ -2122,6 +2139,17 @@ void disconnect_bsp_APIC(int virt_wire_setup)
>       apic_write(APIC_LVT1, value);
>   }
>
> +static int get_cpunum(int apicid)
> +{
> +    int cpu;
> +
> +    cpu = apicid_to_cpunum[apicid];
> +    if (cpu < 0)
> +        cpu = cpumask_next_zero(-1, cpu_number_mask);
> +
> +    return cpu;
> +}
> +
>   int generic_processor_info(int apicid, int version)
>   {
>       int cpu, max = nr_cpu_ids;
> @@ -2199,7 +2227,9 @@ int generic_processor_info(int apicid, int version)
>            */
>           cpu = 0;
>       } else
> -        cpu = cpumask_next_zero(-1, cpu_present_mask);
> +        cpu = get_cpunum(apicid);
> +
> +    apicid_to_cpunum[apicid] = cpu;
>
>       /*
>        * Validate version
> @@ -2228,6 +2258,7 @@ int generic_processor_info(int apicid, int version)
>       early_per_cpu(x86_cpu_to_logical_apicid, cpu) =
>           apic->x86_32_early_logical_apicid(cpu);
>   #endif
> +    cpumask_set_cpu(cpu, cpu_number_mask);
>       set_cpu_possible(cpu, true);
>       set_cpu_present(cpu, true);
>
> --
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