netdev - Re: [PATCH v6] lib: optimize cpumask_local

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Date:   Fri, 13 Nov 2020 08:02:29 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Shaokun Zhang <zhangshaokun@...ilicon.com>,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Cc:     Yuqi Jin <jinyuqi@...wei.com>,
        Rusty Russell <rusty@...tcorp.com.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Juergen Gross <jgross@...e.com>,
        Paul Burton <paul.burton@...s.com>,
        Michal Hocko <mhocko@...e.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [PATCH v6] lib: optimize cpumask_local_spread()

On 11/12/20 6:06 PM, Shaokun Zhang wrote:
>>> On Huawei Kunpeng 920 server, there are 4 NUMA node(0 - 3) in the 2-cpu
>>> system(0 - 1). The topology of this server is followed:
>>
>> This is with a feature enabled that Intel calls sub-NUMA-clustering
>> (SNC), right?  Explaining *that* feature would also be great context for
> 
> Correct,
> 
>> why this gets triggered on your system and not normally on others and
>> why nobody noticed this until now.
> 
> This is on intel 6248 platform:

I have no idea what a "6248 platform" is.

>>> +static void calc_node_distance(int *node_dist, int node)
>>> +{
>>> +	int i;
>>> +
>>> +	for (i = 0; i < nr_node_ids; i++)
>>> +		node_dist[i] = node_distance(node, i);
>>> +}
>>
>> This appears to be the only place node_dist[] is written.  That means it
>> always contains a one-dimensional slice of the two-dimensional data
>> represented by node_distance().
>>
>> Why is a copy of this data needed?
> 
> It is used to store the distance with the @node for later, apologies that I
> can't follow your question correctly.

Right, the data that you store is useful.  *But*, it's also a verbatim
copy of the data from node_distance().  Why not just use node_distance()
directly in your code rather than creating a partial copy of it in the
local node_dist[] array?


>>>  unsigned int cpumask_local_spread(unsigned int i, int node)
>>>  {
>>> -	int cpu, hk_flags;
>>> +	static DEFINE_SPINLOCK(spread_lock);
>>> +	static int node_dist[MAX_NUMNODES];
>>> +	static bool used[MAX_NUMNODES];
>>
>> Not to be *too* picky, but there is a reason we declare nodemask_t as a
>> bitmap and not an array of bools.  Isn't this just wasteful?
>>
>>> +	unsigned long flags;
>>> +	int cpu, hk_flags, j, id;
>>>  	const struct cpumask *mask;
>>>  
>>>  	hk_flags = HK_FLAG_DOMAIN | HK_FLAG_MANAGED_IRQ;
>>> @@ -220,20 +256,28 @@ unsigned int cpumask_local_spread(unsigned int i, int node)
>>>  				return cpu;
>>>  		}
>>>  	} else {
>>> -		/* NUMA first. */
>>> -		for_each_cpu_and(cpu, cpumask_of_node(node), mask) {
>>> -			if (i-- == 0)
>>> -				return cpu;
>>> -		}
>>> +		spin_lock_irqsave(&spread_lock, flags);
>>> +		memset(used, 0, nr_node_ids * sizeof(bool));
>>> +		calc_node_distance(node_dist, node);
>>> +		/* Local node first then the nearest node is used */
>>
>> Is this comment really correct?  This makes it sound like there is only
> 
> I think it is correct, that's what we want to choose the nearest node.
> 
>> fallback to a single node.  Doesn't the _code_ fall back basically
>> without limit?
> 
> If I follow your question correctly, without this patch, if the local
> node is used up, one random node will be choosed, right? Now we firstly
> choose the nearest node by the distance, if all nodes has been choosen,
> it will return the initial solution.

The comment makes it sound like the code does:
	1. Do the local node
	2. Do the next nearest node
	3. Stop

In reality, I *think* it's more of a loop where it search
ever-increasing distances away from the local node.

I just think the comment needs to be made more precise.

>>> +		for (j = 0; j < nr_node_ids; j++) {
>>> +			id = find_nearest_node(node_dist, used);
>>> +			if (id < 0)
>>> +				break;
>>>  
>>> -		for_each_cpu(cpu, mask) {
>>> -			/* Skip NUMA nodes, done above. */
>>> -			if (cpumask_test_cpu(cpu, cpumask_of_node(node)))
>>> -				continue;
>>> +			for_each_cpu_and(cpu, cpumask_of_node(id), mask)
>>> +				if (i-- == 0) {
>>> +					spin_unlock_irqrestore(&spread_lock,
>>> +							       flags);
>>> +					return cpu;
>>> +				}
>>> +			used[id] = 1;
>>> +		}
>>> +		spin_unlock_irqrestore(&spread_lock, flags);
>>
>> The existing code was pretty sparsely commented.  This looks to me to
>> make it more complicated and *less* commented.  Not the best combo.
> 
> Apologies for the bad comments, hopefully I describe it clearly by the above
> explantion.

Do you want to take another pass at submitting this patch?