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Date: Thu, 17 Jul 2014 09:07:46 +0900
From: Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
To: Borislav Petkov <bp@...en8.de>, Tony Luck <tony.luck@...el.com>
CC: <tglx@...utronix.de>, <mingo@...hat.com>, <hpa@...or.com>,
<x86@...nel.org>, <toshi.kani@...com>, <imammedo@...hat.com>,
<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 v2] x86,cpu-hotplug: assign same CPU number to readded
CPU
(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@...fujitsu.com>
>> Suggested-by: Borislav Petkov <bp@...en8.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);
>>
>>
>>
>
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