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

[Wanpeng Li <kernellwp@gmail.com>]

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@hp.com>
>> Reviewed-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
>> Tested-by: Linn Crosetto <linn@hp.com>
>> Signed-off-by: Wanpeng Li <wanpeng.li@linux.intel.com>
> 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

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.

Regards,
Wanpeng Li

> Thanks,
> -Kame
>
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