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Date:	Thu, 10 Jul 2014 11:26:09 +0200
From:	Igor Mammedov <imammedo@...hat.com>
To:	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
Cc:	<tglx@...utronix.de>, <mingo@...hat.com>, <hpa@...or.com>,
	<x86@...nel.org>, <toshi.kani@...com>, <bp@...en8.de>,
	<huawei.libin@...wei.com>, <paul.gortmaker@...driver.com>,
	<linux-kernel@...r.kernel.org>, <srivatsa.bhat@...ux.vnet.ibm.com>,
	<umgwanakikbuti@...il.com>, <peterz@...radead.org>
Subject: Re: [PATCH] x86,cpu-hotplug: assign same CPU number to readded CPU

On Thu, 10 Jul 2014 17:41:50 +0900
Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com> 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@...fujitsu.com>
> Suggested-by: Borislav Petkov <bp@...en8.de1>
> ---
>   arch/x86/kernel/apic/apic.c | 32 +++++++++++++++++++++++++++++++-
>   1 file changed, 31 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
> index ad28db7..1cc715b 100644
> --- a/arch/x86/kernel/apic/apic.c
> +++ b/arch/x86/kernel/apic/apic.c
> @@ -220,6 +220,22 @@ static void apic_pm_activate(void);
>   static unsigned long apic_phys;
> 
>   /*
> + * Bind ACPI ID to CPU ID
> + * CPU ID 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_cpuid[MAX_LOCAL_APIC] = {
> +	[0 ... MAX_LOCAL_APIC-1] = -1,
> +};
> +
> +/*
> + * Represent CPU ID bound to APIC
> + * Don't clear a bit even if CPU is hot-removed
> + */
> +static DECLARE_BITMAP(cpu_used_bits, CONFIG_NR_CPUS);
> +static struct cpumask *const cpu_used_mask = to_cpumask(cpu_used_bits);
> +
> +/*
>    * Get the LAPIC version
>    */
>   static inline int lapic_get_version(void)
> @@ -2122,6 +2138,17 @@ void disconnect_bsp_APIC(int virt_wire_setup)
>   	apic_write(APIC_LVT1, value);
>   }
> 
> +static int get_cpuid(int apicid)
> +{
> +	int cpuid;
> +
> +	cpuid = apicid_to_cpuid[apicid];
> +	if (cpuid < 0)
> +		cpuid = cpumask_next_zero(-1, cpu_used_mask);
Why do you need additional cpu bitmask?
How about just finding the first apicid_to_cpuid[apicid] < 0
and dropping not needed anymore bitmask.

> +
> +	return cpuid;
> +}
> +
>   int generic_processor_info(int apicid, int version)
>   {
>   	int cpu, max = nr_cpu_ids;
> @@ -2199,7 +2226,9 @@ int generic_processor_info(int apicid, int version)
>   		 */
>   		cpu = 0;
>   	} else
> -		cpu = cpumask_next_zero(-1, cpu_present_mask);
> +		cpu = get_cpuid(apicid);
> +
> +	apicid_to_cpuid[apicid] = cpu;
> 
>   	/*
>   	 * Validate version
> @@ -2228,6 +2257,7 @@ 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_used_mask);
>   	set_cpu_possible(cpu, true);
>   	set_cpu_present(cpu, true);
> 
> 

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