Re: [PATCH v2] x86,cpu-hotplug: assign same CPU number to readded CPU

[Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>]

(2014/07/16 20:11), Borislav Petkov wrote:
> On Wed, Jul 16, 2014 at 04:33:03PM +0900, 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@jp.fujitsu.com>
>> Suggested-by: Borislav Petkov <bp@alien8.de1>
>
> You can remove the "1" above as the domain is .de :-)

Sorry for my mistake. I'll update soon.

>
> Ok, so after looking at the dmesg you sent me, it looks like it works as
> expected.
>
> Apparently, the cores get enumerated in the order they're listed in SRAT
> - this also explains the mapping between APIC ID and core number in
> Linux.
>
> Your patch looks simple enough as you're basically making that mapping
> explicit with apicid_to_cpunum and make sure it remains stable when the
> cores reappear.
>
> And since APIC ID doesn't change across physical hotplug (does it?), it
> all works as expected.
>
> But, since this is BIOS and BIOS does crazy hacks to accomodate b0rked
> OSes, I'd like it if Tony looked at this too, whether it makes sense and
> whether that solution is fine.
>
> @Tony: the text should explain it all, leaving in the rest.

Thank you for reviewing my patch and confirming dmesg.
I wait Tony's comment.

Thanks,
Yasuaki Ishimatsu

>
> Thanks.
>
>> ---
>> v2: change cpuid to cpunum
>> ---
>>   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..5dc3e50 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 ACPI 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_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 +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_used_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_used_mask);
>>   	set_cpu_possible(cpu, true);
>>   	set_cpu_present(cpu, true);
>>
>>
>>
>