[PATCH v2 0/7] arm64: numa/topology/smp: update the cpumasks for CPU hotplug

[sudeep.holla@arm.com (Sudeep Holla)]

Hi Hanjun,

On 27/06/18 04:51, Hanjun Guo wrote:
> On 2018/6/26 17:23, Sudeep Holla wrote:
> [...]
>>> Core(s) per socket:    15      //it's 15 now...
>>
>> I know exactly what's the problem. And I think that's exactly the
>> behavior on my x86 box too. Further you might see another issue
>> as most of these applications like lstopo and lscpu parse only CPU0
>> sysfs, things fall apart if CPU0 is hotplugged out which is not allowed
>> on most of x86 boxes I have tried.
>>
>> So, for me, that's issue with the application TBH.
>>
>> [..]
>>
>>> Offlined 16 cores on NUMA node 0, everything backs to normal:
>>
>> Expected :)
>>
>>> but when onlined CPU15 after,
>>> Architecture:          aarch64
>>> Byte Order:            Little Endian
>>> CPU(s):                64
>>> On-line CPU(s) list:   15-63
>>> Off-line CPU(s) list:  0-14
>>> Thread(s) per core:    1
>>> Core(s) per socket:    12     // it's 12, hmm...
>>
>> I had to looks at the lscpu code and I think that explains why you can
>> get 12 here. We have
>> 	add_summary_n(tb, _("Core(s) per socket:"),
>> 		cores_per_socket ?: desc->ncores / desc->nsockets);
>>
>> Now cores_per_socket because we don't have procfs entry, so ncores=49
>> and nsockets=4, which means you get 12. TBH lscpu should be used only
>> when all CPUs are online. Lots of inaccurate assumptions in it.
>>
>>>
>>> I think maybe the cpumask for the socket is changed in a
>>> wrong way when online/offline CPUs, didn't take a deep look into
>>> that, hope the test helps.
>>>
>>
>> I am not convinced if there's any issue unless you see discrepancy in
>> sysfs entries themselves and not though applications interpreting the
>> values for you based on some wrong assumptions.
> 
> Thanks for the clarify and let me know the detail, I tested 4.17 on
> a x86 machine which CPU0 can be offlined, also with the same lscpu version,
> I got the same on x86:
> 

Thanks for taking trouble and confirming this.

> lscpu
> Architecture:          x86_64
> CPU op-mode(s):        32-bit, 64-bit
> Byte Order:            Little Endian
> CPU(s):                72
> On-line CPU(s) list:   1-9,11-23,25-45,47-65,67-71
> Off-line CPU(s) list:  0,10,24,46,66                  // offline CPUs to make no continuous 18 logical CPUs on a socket
> Thread(s) per core:    1                              // it's 2 before CPU offlined
> Core(s) per socket:    17                             // and not 18 which is the value before CPU offlined
> Socket(s):             2
> NUMA node(s):          2
> Vendor ID:             GenuineIntel
> CPU family:            6
> Model:                 85
> Model name:            Intel(R) Xeon(R) Gold 6140 CPU @ 2.30GHz
> Stepping:              4
> CPU MHz:               2999.755
> CPU max MHz:           3700.0000
> CPU min MHz:           1000.0000
> BogoMIPS:              4600.00
> Virtualization:        VT-x
> L1d cache:             32K
> L1i cache:             32K
> L2 cache:              1024K
> L3 cache:              25344K
> NUMA node0 CPU(s):     1-9,11-17,36-45,47-53
> NUMA node1 CPU(s):     18-23,25-35,54-65,67-71
> 
> So with this patch set:
> 
> Tested-by: Hanjun Guo <hanjun.guo@linaro.org>
> 

Thanks for testing.

-- 
Regards,
Sudeep