linux-kernel - Re: [PATCH v11 5/8] mm/demotion: Build demotion targets based on explicit memory tiers

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Date:   Fri, 29 Jul 2022 12:52:13 +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 v11 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:
> + */

....

>> +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);
>
> Don't find code to optimize node_random() for weight == 1 case, forget
> to do that?

I guess you suggested to do that as the patch for node_random or did I
got the review feedback wrong?

https://lore.kernel.org/linux-mm/87y1wdn30p.fsf@yhuang6-desk2.ccr.corp.intel.com

The change for node_random will be patch outside this series.

-aneesh