Re: [PATCH v5] numa: make node_to_cpumask_map() NUMA_NO_NODE aware

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2019/9/17 18:08, Michal Hocko wrote:
> On Tue 17-09-19 17:53:57, Yunsheng Lin wrote:
>> On 2019/9/17 17:36, Michal Hocko wrote:
>>> On Tue 17-09-19 14:20:11, Yunsheng Lin wrote:
>>>> On 2019/9/17 13:28, Michael Ellerman wrote:
>>>>> Yunsheng Lin <linyunsheng@huawei.com> writes:
>>> [...]
>>>>>> But we cannot really copy the page allocator logic. Simply because the
>>>>>> page allocator doesn't enforce the near node affinity. It just picks it
>>>>>> up as a preferred node but then it is free to fallback to any other numa
>>>>>> node. This is not the case here and node_to_cpumask_map will only restrict
>>>>>> to the particular node's cpus which would have really non deterministic
>>>>>> behavior depending on where the code is executed. So in fact we really
>>>>>> want to return cpu_online_mask for NUMA_NO_NODE.
>>>>>>
>>>>>> Some arches were already NUMA_NO_NODE aware, but they return cpu_all_mask,
>>>>>> which should be identical with cpu_online_mask when those arches do not
>>>>>> support cpu hotplug, this patch also changes them to return cpu_online_mask
>>>>>> in order to be consistent and use NUMA_NO_NODE instead of "-1".
>>>>>
>>>>> Except some of those arches *do* support CPU hotplug, powerpc and sparc
>>>>> at least. So switching from cpu_all_mask to cpu_online_mask is a
>>>>> meaningful change.
>>>>
>>>> Yes, thanks for pointing out.
>>>>
>>>>>
>>>>> That doesn't mean it's wrong, but you need to explain why it's the right
>>>>> change.
>>>>
>>>> How about adding the below to the commit log:
>>>> Even if some of the arches do support CPU hotplug, it does not make sense
>>>> to return the cpu that has been hotplugged.
>>>>
>>>> Any suggestion?
>>>
>>> Again, for the third time, I believe. Make it a separate patch please.
>>> There is absolutely no reason to conflate those two things.
>>
>> Ok, thanks.
>> Will make the cpu_all_mask -> cpu_online_mask change a separate patch.
> 
> Thanks. This really needs per arch maintainer to check closely.
> 
>> Also, do you think it is better to resend this as individual patches for each arch
>> or have all these changes in a single patch? I am not sure which is the common
>> practice for a multi-arches changes like this.
> 
> It really depends on arch maintainers. Both approaches have some pros
> and cons. A single patch is more compact and and parts are not going to
> get lost in noise. They might generate some conflicts with parallel
> changes. I suspect a conflict risk is quite low in this code considering
> from a recent activity. A follow up arch specific patch would have to be
> routed via Andrew as well.
> 
> If Andrew is ok routing it through his tree and none of the arch
> maintainers is opposed then I would go with a single patch.

Ok, I will try a single patch for NUMA_NO_NODE aware change first.
"cpu_all_mask -> cpu_online_mask" change seems a little controversial,
and may need deeper investigation.


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