linux-kernel - Re: [PATCH v5 2/9] mm/demotion: Expose per node memory tier to sysfs

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Date:   Wed, 8 Jun 2022 10:25:09 +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 2/9] mm/demotion: Expose per node memory tier to sysfs

On 6/8/22 1:45 AM, Tim Chen wrote:
> On Fri, 2022-06-03 at 19:12 +0530, Aneesh Kumar K.V wrote:
>>
>>   
>> +static struct memory_tier *__node_get_memory_tier(int node)
>> +{
>> +	struct memory_tier *memtier;
>> +
>> +	list_for_each_entry(memtier, &memory_tiers, list) {
> 
> We could need to map node to mem_tier quite often, if we need
> to account memory usage at tier level.  It will be more efficient
> to have a pointer from node (pgdat) to memtier rather
> than doing a search through the list.
> 
> 

That is something I was actively trying to avoid. Currently all struct 
memory_tier references are with memory_tier_lock mutex held. That 
simplify the locking and reference counting.

As of now we are able to implement all the required interfaces without 
pgdat having pointers to struct memory_tier. We can update pgdat with 
memtier details when we are implementing changes requiring those. We 
could keep additional memtier->dev reference to make sure memory tiers 
are not destroyed while other part of the kernel is referencing the 
same. But IMHO such changes should wait till we have users for the same.


>> +		if (node_isset(node, memtier->nodelist))
>> +			return memtier;
>> +	}
>> +	return NULL;
>> +}
>> +
>>
> 
> Tim
>

-aneesh