Re: [PATCH v5] x86, cpu-hotplug: fix llc shared map unreleased during cpu hotplug

[Kamezawa Hiroyuki]

(2014/09/23 15:36), Wanpeng Li wrote:
> Hi Kamezawa,
> 于 14-9-23 下午12:46, Kamezawa Hiroyuki 写道:
>> (2014/09/17 16:17), 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 repeatedly.
>>>
>>> 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.
>>>
>>> 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>
>> Yasuaki reported this can happen on our real hardware.
>> https://lkml.org/lkml/2014/7/22/1018
>>
>> Our 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.
>> ==
>>
>> So, please.
> 
> As I mentioned before, we still observe calltrace after Yasuaki's patch
> applied.
> https://lkml.org/lkml/2014/7/29/40
> 
Yes. 
I just wanted to say we need your patch by showing real hardware case.
Sorry for confusion I just reused his explanation of the problem.

I know Yasuaki's original trial was clearing llc_shared map as you do.

> Actually I prefer to merge both patches, one for fix llc shared map
> unreleased during hotplug and the other one for assign same CPU number
> to readded CPU.
> 
I agree. 

Thanks,
-Kame

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