linux-kernel - Re: [PATCH v10 5/8] mm/demotion: Build demotion targets based on explicit memory tiers

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Date:   Wed, 27 Jul 2022 10:05:50 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        Wei Xu <weixugc@...gle.com>, Yang Shi <shy828301@...il.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim C Chen <tim.c.chen@...el.com>,
        Michal Hocko <mhocko@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Hesham Almatary <hesham.almatary@...wei.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Alistair Popple <apopple@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Johannes Weiner <hannes@...xchg.org>, jvgediya.oss@...il.com
Subject: Re: [PATCH v10 5/8] mm/demotion: Build demotion targets based on
 explicit memory tiers

"Huang, Ying" <ying.huang@...el.com> writes:

> Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com> writes:
>
>> On 7/26/22 1:14 PM, Huang, Ying wrote:
>>> "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com> writes:
>>> 

....

>>> + */
>>>> +int next_demotion_node(int node)
>>>> +{
>>>> +	struct demotion_nodes *nd;
>>>> +	int target;
>>>> +
>>>> +	if (!node_demotion)
>>>> +		return NUMA_NO_NODE;
>>>> +
>>>> +	nd = &node_demotion[node];
>>>> +
>>>> +	/*
>>>> +	 * node_demotion[] is updated without excluding this
>>>> +	 * function from running.
>>>> +	 *
>>>> +	 * Make sure to use RCU over entire code blocks if
>>>> +	 * node_demotion[] reads need to be consistent.
>>>> +	 */
>>>> +	rcu_read_lock();
>>>> +	/*
>>>> +	 * If there are multiple target nodes, just select one
>>>> +	 * target node randomly.
>>>> +	 *
>>>> +	 * In addition, we can also use round-robin to select
>>>> +	 * target node, but we should introduce another variable
>>>> +	 * for node_demotion[] to record last selected target node,
>>>> +	 * that may cause cache ping-pong due to the changing of
>>>> +	 * last target node. Or introducing per-cpu data to avoid
>>>> +	 * caching issue, which seems more complicated. So selecting
>>>> +	 * target node randomly seems better until now.
>>>> +	 */
>>>> +	target = node_random(&nd->preferred);
>>> 
>>> In one of the most common cases, nodes_weight(&nd->preferred) == 1.
>>> Where, get_random_int() in node_random() just wastes CPU cycles and
>>> random entropy.  So the original struct demotion_nodes implementation
>>> appears better.
>>> 
>>>   struct demotion_nodes {
>>>          unsigned short nr;
>>>          short nodes[DEMOTION_TARGET_NODES];
>>>   };
>>> 
>>
>>
>> Is that measurable difference? using nodemask_t makes it much easier with respect to
>> implementation. IMHO if we observe the usage of node_random() to have performance impact
>> with nodes_weight() == 1 we should fix node_random() to handle that? If you strongly
>> feel we should fix this, i can opencode node_random to special case node_weight() == 1?
>
> If there's no much difference, why not just use the existing code?
> IMHO, it's your responsibility to prove your new implementation is
> better via numbers, for example, reduced code lines, with better or same
> performance.
>
> Another policy is just to use the existing code in the first version.
> Then change it based on measurement.

One of the reason I switched to nodemask_t is to make code simpler.
demotion target is essentially a node mask. 

>
> In general, I care more about the most common cases, that is, 0 or 1
> demotion target.

How about I switch to the below opencoded version. That should take care
of the above concern. 

>
>> -	target = node_random(&nd->preferred);
>> +	node_weight = nodes_weight(nd->preferred);
>> +	switch (node_weight) {
>> +	case 0:
>> +		target = NUMA_NO_NODE;
>> +		break;
>> +	case 1:
>> +		target = first_node(nd->preferred);
>> +		break;
>> +	default:
>> +		target = bitmap_ord_to_pos(nd->preferred.bits,
>> +					   get_random_int() % node_weight, MAX_NUMNODES);
>> +		break;
>> +	}
>>  
>>