Re: [mm/migrate] 9eeb73028c: stress-ng.memhotplug.ops_per_sec -53.8% regression

Huang, Ying
2021-09-06T05:58:14Z

Dave Hansen <dave.hansen@intel.com> writes:

> On 9/5/21 6:53 PM, Huang, Ying wrote:
>>> in testcase: stress-ng
>>> on test machine: 96 threads 2 sockets Intel(R) Xeon(R) Gold 6252 CPU @ 2.10GHz with 192G memory
>>> with following parameters:
>>>
>>> nr_threads: 10%
>>> disk: 1HDD
>>> testtime: 60s
>>> fs: ext4
>>> class: os
>>> test: memhotplug
>>> cpufreq_governor: performance
>>> ucode: 0x5003006
>>>
>> Because we added some operations during online/offline CPU, it's
>> expected that the performance of online/offline CPU will decrease. In
>> most cases, the performance of CPU hotplug isn't a big problem. But
>> then I remembers that the performance of the CPU hotplug may influence
>> suspend/resume performance :-(
>>
>> It appears that it is easy and reasonable to enclose the added
>> operations inside #ifdef CONFIG_NUMA. Is this sufficient to restore the
>> performance of suspend/resume?
>
> It's "memhotplug", not CPUs, right?

Yes. Thanks for pointing that out!

We will update node_demotion[] in CPU hotplug too. Because the status
that whether a node has CPU may change after CPU hotplug. And CPU
online/offline performance may be relevant for suspend/resume.

> I didn't do was to actively go out and look for changes that would
> affect the migration order. The code just does regenerates and writes
> the order blindly when it sees any memory hotplug event. I have the
> feeling the synchronize_rcu()s are what's killing us.
>
> It would be pretty easy to go and generate the order, but only do the
> update and the RCU bits when the order changes from what was there.
>
> I guess we have a motivation now.

I don't know whether the performance of memory hotplug is important or
not. But it should be welcome not to make it too bad. You proposal
sounds good.

Best Regards,
Huang, Ying