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Message-ID: <6a2b05db-8e8d-b5e0-6166-f5c41c410c7b@linux.ibm.com>
Date:   Wed, 8 Jun 2022 10:32:26 +0530
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Subject: Re: [PATCH v5 4/9] mm/demotion: Build demotion targets based on
 explicit memory tiers
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To:     Tim Chen <tim.c.chen@linux.intel.com>, linux-mm@kvack.org,
        akpm@linux-foundation.org
Cc:     Wei Xu <weixugc@google.com>, Huang Ying <ying.huang@intel.com>,
        Greg Thelen <gthelen@google.com>,
        Yang Shi <shy828301@gmail.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Tim C Chen <tim.c.chen@intel.com>,
        Brice Goglin <brice.goglin@gmail.com>,
        Michal Hocko <mhocko@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Hesham Almatary <hesham.almatary@huawei.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        Alistair Popple <apopple@nvidia.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Feng Tang <feng.tang@intel.com>,
        Jagdish Gediya <jvgediya@linux.ibm.com>,
        Baolin Wang <baolin.wang@linux.alibaba.com>,
        David Rientjes <rientjes@google.com>
References: <20220603134237.131362-1-aneesh.kumar@linux.ibm.com>
 <20220603134237.131362-5-aneesh.kumar@linux.ibm.com>
 <c4678658cdd04d14ced7d0407da32f5fdec19f95.camel@linux.intel.com>
From:   Aneesh Kumar K V <aneesh.kumar@linux.ibm.com>
In-Reply-To: <c4678658cdd04d14ced7d0407da32f5fdec19f95.camel@linux.intel.com>
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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