linux-kernel - Re: [PATCH v5 9/9] mm/demotion: Update node_is

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Date:   Mon, 6 Jun 2022 14:03:21 +0530
From:   Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>
To:     Ying Huang <ying.huang@...el.com>, linux-mm@...ck.org,
        akpm@...ux-foundation.org
Cc:     Wei Xu <weixugc@...gle.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 9/9] mm/demotion: Update node_is_toptier to work with
 memory tiers

On 6/6/22 12:54 PM, Ying Huang wrote:
> On Mon, 2022-06-06 at 09:22 +0530, Aneesh Kumar K V wrote:
>> On 6/6/22 8:41 AM, Ying Huang wrote:
>>> On Fri, 2022-06-03 at 19:12 +0530, Aneesh Kumar K.V wrote:
>>>> With memory tiers support we can have memory on NUMA nodes
>>>> in the top tier from which we want to avoid promotion tracking NUMA
>>>> faults. Update node_is_toptier to work with memory tiers. To
>>>> avoid taking locks, a nodemask is maintained for all demotion
>>>> targets. All NUMA nodes are by default top tier nodes and as
>>>> we add new lower memory tiers NUMA nodes get added to the
>>>> demotion targets thereby moving them out of the top tier.
>>>
>>> Check the usage of node_is_toptier(),
>>>
>>> - migrate_misplaced_page()
>>>     node_is_toptier() is used to check whether migration is a promotion.
>>> We can avoid to use it.  Just compare the rank of the nodes.
>>>
>>> - change_pte_range() and change_huge_pmd()
>>>     node_is_toptier() is used to avoid scanning fast memory (DRAM) pages
>>> for promotion.  So I think we should change the name to node_is_fast()
>>> as follows,
>>>
>>> static inline bool node_is_fast(int node)
>>> {
>>> 	return NODE_DATA(node)->mt_rank >= MEMORY_RANK_DRAM;
>>> }
>>>
>>
>> But that gives special meaning to MEMORY_RANK_DRAM. As detailed in other
>> patches, absolute value of rank doesn't carry any meaning. It is only
>> the relative value w.r.t other memory tiers that decide whether it is
>> fast or not. Agreed by default memory tiers get built with
>> MEMORY_RANK_DRAM. But userspace can change the rank value of 'memtier1'
>> Hence to determine a node is consisting of fast memory is essentially
>> figuring out whether node is the top most tier in memory hierarchy and
>> not just the memory tier rank value is >= MEMORY_RANK_DRAM?
> 
> In a system with 3 tiers,
> 
> HBM	0
> DRAM	1
> PMEM	2
> 
> In your implementation, only HBM will be considered fast.  But what we
> need is to consider both HBM and DRAM fast.  Because we use NUMA
> balancing to promote PMEM pages to DRAM.  It's unnecessary to scan HBM
> and DRAM pages for that.  And there're no requirements to promote DRAM
> pages to HBM with NUMA balancing.
> 
> I can understand that the memory tiers are more dynamic now.  For
> requirements of NUMA balancing, we need the lowest memory tier (rank)
> where there's at least one node with CPU.  The nodes in it and the
> higher tiers will be considered fast.
> 

is this good (not tested)?
/*
  * build the allowed promotion mask. Promotion is allowed
  * from higher memory tier to lower memory tier only if
  * lower memory tier doesn't include compute. We want to
  * skip promotion from a memory tier, if any node which is
  * part of that memory tier have CPUs. Once we detect such
  * a memory tier, we consider that tier as top tier from
  * which promotion is not allowed.
  */
list_for_each_entry_reverse(memtier, &memory_tiers, list) {
	nodes_and(allowed, node_state[N_CPU], memtier->nodelist);
	if (nodes_empty(allowed))
		nodes_or(promotion_mask, promotion_mask, allowed);
	else
		break;
}

and then

static inline bool node_is_toptier(int node)
{

	return !node_isset(node, promotion_mask);
}