linux-kernel - Re: [PATCH v3] x86,cpu-hotplug: assign same CPU number to readded CPU

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Message-ID: <53CF1DE3.7090001@jp.fujitsu.com>
Date:	Wed, 23 Jul 2014 11:28:51 +0900
From:	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
To:	Toshi Kani <toshi.kani@...com>
CC:	<tglx@...utronix.de>, <mingo@...hat.com>, <hpa@...or.com>,
	<bp@...en8.de>, <gong.chen@...ux.intel.com>, <tony.luck@...el.com>,
	<x86@...nel.org>, <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 v3] x86,cpu-hotplug: assign same CPU number to readded
 CPU

(2014/07/23 3:29), Toshi Kani wrote:
> On Thu, 2014-07-17 at 16:32 +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.de>
>
>> ---
>> v2: change cpuid to cpunum
>> v3: fix Borislav's email address of Suggested-by
>>       fix typo (ACPI ID to APIC ID)
>> ---
>>    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 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_used_bits, CONFIG_NR_CPUS);
>> +static struct cpumask *const cpu_used_mask = to_cpumask(cpu_used_bits);
>
> I think the name of cpu_used_mask is confusing since the term "used"
> sounds some state of CPU, which is not the case here.  How about
> cpu_number_mask?  This is more specific and avoids such confusion.

I'll update it soon.

>
> Otherwise, the change looks good.
>
> Reviewed-by: Toshi Kani <toshi.kani@...com>

Thank you for review.

Thanks,
Yasuaki Ishimatsu

>
> Thanks,
> -Toshi
>
>> +
>> +/*
>>     * Get the LAPIC version
>>     */
>    ^^
> ps. I had a problem in applying this patch since there was an extra
> blank character at the begging of unchanged lines.  But this might be an
> issue in my mailer.
>
>


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