Re: [PATCH v6] sched: fix llc shared map unreleased during cpu hotplug

[Wanpeng Li]

于 9/24/14, 4:24 PM, Yasuaki Ishimatsu 写道:
> (2014/09/24 17:13), Wanpeng Li wrote:
>> BUG: unable to handle kernel NULL pointer dereference at 0000000000000004
>> IP: [..] find_busiest_group
>> PGD 5a9d5067 PUD 13067 PMD 0
>> Oops: 0000 [#3] SMP
>> [...]
>> Call Trace:
>> load_balance
>> ? _raw_spin_unlock_irqrestore
>> idle_balance
>> __schedule
>> schedule
>> schedule_timeout
>> ? lock_timer_base
>> schedule_timeout_uninterruptible
>> msleep
>> lock_device_hotplug_sysfs
>> online_store
>> dev_attr_store
>> sysfs_write_file
>> vfs_write
>> SyS_write
>> system_call_fastpath
>>
>> This bug can be triggered by hot add and remove large number of xen
>> domain0's vcpus repeatly.
>>
>> Last level cache shared map is built during cpu up and build sched domain
>> routine takes advantage of it to setup sched domain cpu topology, however,
>> llc shared map is unreleased during cpu disable which lead to invalid sched
>> domain cpu topology. This patch fix it by release llc shared map correctly
>> during cpu disable.
>>
>> Yasuaki also reported this can happen on their real hardware.
>> https://lkml.org/lkml/2014/7/22/1018
>>
>> His case is here.
>> ==
>> 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.
> Please remove it. The description is not explanation for your patch.

Ok， ；-）

Regards,
Wanpeng Li

>
> Thanks,
> Yasuaki Ishimatsu
>
>> Reviewed-by: Borislav Petkov <bp@xxxxxxx>
>> Reviewed-by: Toshi Kani <toshi.kani@xxxxxx>
>> Reviewed-by: Yasuaki Ishimatsu <isimatu.yasuaki@xxxxxxxxxxxxxx>
>> Tested-by: Linn Crosetto <linn@xxxxxx>
>> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxxxxxx>
>> ---
>> v5 -> v6:
>>   * add the real-hardware reports to the changelog
>> v4 -> v5:
>>   * add the description when the bug can occur
>> v3 -> v4:
>>   * simplify backtrace
>> v2 -> v3:
>>   * simplify backtrace
>> v1 -> v2:
>>   * fix subject line
>>
>>   arch/x86/kernel/smpboot.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
>> index 5492798..0134ec7 100644
>> --- a/arch/x86/kernel/smpboot.c
>> +++ b/arch/x86/kernel/smpboot.c
>> @@ -1292,6 +1292,9 @@ static void remove_siblinginfo(int cpu)
>>   
>>   	for_each_cpu(sibling, cpu_sibling_mask(cpu))
>>   		cpumask_clear_cpu(cpu, cpu_sibling_mask(sibling));
>> +	for_each_cpu(sibling, cpu_llc_shared_mask(cpu))
>> +		cpumask_clear_cpu(cpu, cpu_llc_shared_mask(sibling));
>> +	cpumask_clear(cpu_llc_shared_mask(cpu));
>>   	cpumask_clear(cpu_sibling_mask(cpu));
>>   	cpumask_clear(cpu_core_mask(cpu));
>>   	c->phys_proc_id = 0;
>>
>
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