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

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Message-ID: <c810947d-26a4-96f9-de01-7f74e64877db@linux.ibm.com>
Date:   Wed, 8 Jun 2022 21:45:43 +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 9:36 PM, Tim Chen wrote:
> On Wed, 2022-06-08 at 10:25 +0530, Aneesh Kumar K V wrote:
>> 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.
>>
> 
> I think we should have an efficient mapping from node to memtier from
> the get go.  There are many easily envisioned scenarios where
> we need to map from node to memtier, which Ying pointed out.
> 

I did an initial implementation here. We need to make sure we can access 
NODE_DATA()->memtier lockless. Can you review the changes here

https://lore.kernel.org/linux-mm/87sfoffcfz.fsf@linux.ibm.com

-aneesh