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

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Date:   Thu, 15 Nov 2018 09:50:58 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Keith Busch <keith.busch@...el.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH 1/7] node: Link memory nodes to their compute nodes

On Thu, Nov 15, 2018 at 7:02 AM Keith Busch <keith.busch@...el.com> 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?

I think that would be more flexible, because the set of initiators
that may have "best" or "local" access to a target may be more than 1.