linux-kernel - Re: [PATCH v5 4/9] mm/demotion: Build demotion targets based on explicit memory tiers

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Message-ID: <6a2b05db-8e8d-b5e0-6166-f5c41c410c7b@linux.ibm.com>
Date:   Wed, 8 Jun 2022 10:32:26 +0530
From:   Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>
To:     Tim Chen <tim.c.chen@...ux.intel.com>, linux-mm@...ck.org,
        akpm@...ux-foundation.org
Cc:     Wei Xu <weixugc@...gle.com>, Huang Ying <ying.huang@...el.com>,
        Greg Thelen <gthelen@...gle.com>,
        Yang Shi <shy828301@...il.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim C Chen <tim.c.chen@...el.com>,
        Brice Goglin <brice.goglin@...il.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>,
        Feng Tang <feng.tang@...el.com>,
        Jagdish Gediya <jvgediya@...ux.ibm.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v5 4/9] mm/demotion: Build demotion targets based on
 explicit memory tiers

On 6/8/22 4:21 AM, Tim Chen wrote:
> On Fri, 2022-06-03 at 19:12 +0530, Aneesh Kumar K.V wrote:
>>
>> +int next_demotion_node(int node)
>> +{
>> +	struct demotion_nodes *nd;
>> +	int target, nnodes, i;
>> +
>> +	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();
>> +
>> +	nnodes = nodes_weight(nd->preferred);
>> +	if (!nnodes)
>> +		return NUMA_NO_NODE;
>> +
>> +	/*
>> +	 * 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.
>> +	 */
>> +	nnodes = get_random_int() % nnodes;
>> +	target = first_node(nd->preferred);
>> +	for (i = 0; i < nnodes; i++)
>> +		target = next_node(target, nd->preferred);
> 
> We can simplify the above 4 lines.
> 
> 	target = node_random(nd->preferred);
> 
> There's still a loop overhead though :(
> 

Will fix in next update.

>> +
>> +	rcu_read_unlock();
>> +
>> +	return target;
>> +}
>> +
>>

-aneesh