Re: [PATCH v2] lib: optimize cpumask_local_spread()

[Shaokun Zhang]

Hi Michal,

On 2019/11/6 15:17, Michal Hocko wrote:
> On Tue 05-11-19 17:33:59, Andrew Morton wrote:
>> On Tue, 5 Nov 2019 08:01:41 +0100 Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>>
>>> On Mon 04-11-19 18:27:48, Shaokun Zhang wrote:
>>>> From: yuqi jin <jinyuqi@xxxxxxxxxx>
>>>>
>>>> In the multi-processor and NUMA system, I/O device may have many numa
>>>> nodes belonging to multiple cpus. When we get a local numa, it is
>>>> better to find the node closest to the local numa node, instead
>>>> of choosing any online cpu immediately.
>>>>
>>>> For the current code, it only considers the local NUMA node and it
>>>> doesn't compute the distances between different NUMA nodes for the
>>>> non-local NUMA nodes. Let's optimize it and find the nearest node
>>>> through NUMA distance. The performance will be better if it return
>>>> the nearest node than the random node.
>>>
>>> Numbers please
>>
>> The changelog had
>>
>> : When Parameter Server workload is tested using NIC device on Huawei
>> : Kunpeng 920 SoC:
>> : Without the patch, the performance is 22W QPS;
>> : Added this patch, the performance become better and it is 26W QPS.
> 
> Maybe it is just me but this doesn't really tell me a lot. What is
> Parameter Server workload? What do I do to replicate those numbers? Is

I will give it better description on it in next version. Since it returns
the nearest node from the non-local node than the random one, no harmless
to others, Right?

> this really specific to the Kunpeng 920 server? What is the usual
> variance of the performance numbers?
> 
>>> [...]
>>>> +/**
>>>> + * cpumask_local_spread - select the i'th cpu with local numa cpu's first
>>>> + * @i: index number
>>>> + * @node: local numa_node
>>>> + *
>>>> + * This function selects an online CPU according to a numa aware policy;
>>>> + * local cpus are returned first, followed by the nearest non-local ones,
>>>> + * then it wraps around.
>>>> + *
>>>> + * It's not very efficient, but useful for setup.
>>>> + */
>>>> +unsigned int cpumask_local_spread(unsigned int i, int node)
>>>> +{
>>>> +	int node_dist[MAX_NUMNODES] = {0};
>>>> +	bool used[MAX_NUMNODES] = {0};
>>>
>>> Ugh. This might be a lot of stack space. Some distro kernels use large
>>> NODE_SHIFT (e.g 10 so this would be 4kB of stack space just for the
>>> node_dist).
>>
>> Yes, that's big.  From a quick peek I suspect we could get by using an
>> array of unsigned shorts here but that might be fragile over time even
>> if it works now?
> 
> Whatever data type we use it will be still quite large to be on the
> stack.
> 
>> Perhaps we could make it a statically allocated array and protect the
>> entire thing with a spin_lock_irqsave()?  It's not a frequently called
>> function.
> 
> This is what I was suggesting in previous review feedback.

Ok, will do it in next version.

Thanks,
Shaokun

>