linux-kernel - Re: [PATCH 1/7] node: Link memory nodes to their compute nodes

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Message-ID: <e0b99866-b98b-3385-d0c9-96d5a1d74a6d@arm.com>
Date:   Mon, 19 Nov 2018 08:22:49 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Keith Busch <keith.busch@...el.com>,
        Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
        linux-mm@...ck.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rafael Wysocki <rafael@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH 1/7] node: Link memory nodes to their compute nodes



On 11/15/2018 08:29 PM, Keith Busch wrote:
> On Thu, Nov 15, 2018 at 05:57:10AM -0800, Matthew Wilcox wrote:
>> On Wed, Nov 14, 2018 at 03:49:14PM -0700, Keith Busch wrote:
>>> Memory-only nodes will often have affinity to a compute node, and
>>> platforms have ways to express that locality relationship.
>>>
>>> A node containing CPUs or other DMA devices that can initiate memory
>>> access are referred to as "memory iniators". A "memory target" is a
>>> node that provides at least one phyiscal address range accessible to a
>>> memory initiator.
>>
>> I think I may be confused here.  If there is _no_ link from node X to
>> node Y, does that mean that node X's CPUs cannot access the memory on
>> node Y?  In my mind, all nodes can access all memory in the system,
>> just not with uniform bandwidth/latency.
> 
> The link is just about which nodes are "local". It's like how nodes have
> a cpulist. Other CPUs not in the node's list can acces that node's memory,
> but the ones in the mask are local, and provide useful optimization hints.
> 
> Would a node mask would be prefered to symlinks?

Having hint for local affinity is definitely a plus but this must provide
the coherency matrix to the user preferably in the form of a nodemask for
each memory target.