Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 27 Nov 2018 10:44:57 -0700
From:   Keith Busch <keith.busch@intel.com>
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux ACPI <linux-acpi@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        "Hansen, Dave" <dave.hansen@intel.com>
Subject: Re: [PATCH 2/7] node: Add heterogenous memory performance
Message-ID: <20181127174457.GB6401@localhost.localdomain>
References: <20181114224921.12123-2-keith.busch@intel.com>
 <20181114224921.12123-3-keith.busch@intel.com>
 <CAPcyv4jNpgzpfG1awrxspTeQ1JOK-4-Wu6Kb6cd6NGY6Atj3cg@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAPcyv4jNpgzpfG1awrxspTeQ1JOK-4-Wu6Kb6cd6NGY6Atj3cg@mail.gmail.com>
User-Agent: Mutt/1.9.1 (2017-09-22)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, Nov 26, 2018 at 11:00:09PM -0800, Dan Williams wrote:
> On Wed, Nov 14, 2018 at 2:53 PM Keith Busch <keith.busch@intel.com> wrote:
> >
> > Heterogeneous memory systems provide memory nodes with latency
> > and bandwidth performance attributes that are different from other
> > nodes. Create an interface for the kernel to register these attributes
> > under the node that provides the memory. If the system provides this
> > information, applications can query the node attributes when deciding
> > which node to request memory.
> >
> > When multiple memory initiators exist, accessing the same memory target
> > from each may not perform the same as the other. The highest performing
> > initiator to a given target is considered to be a local initiator for
> > that target. The kernel provides performance attributes only for the
> > local initiators.
> >
> > The memory's compute node should be symlinked in sysfs as one of the
> > node's initiators.
> >
> > The following example shows the new sysfs hierarchy for a node exporting
> > performance attributes:
> >
> >   # tree /sys/devices/system/node/nodeY/initiator_access
> >   /sys/devices/system/node/nodeY/initiator_access
> >   |-- read_bandwidth
> >   |-- read_latency
> >   |-- write_bandwidth
> >   `-- write_latency
> 
> With the expectation that there will be nodes that are initiator-only,
> target-only, or both I think this interface should indicate that. The
> 1:1 "local" designation of HMAT should not be directly encoded in the
> interface, it's just a shortcut for finding at least one initiator in
> the set that can realize the advertised performance. At least if the
> interface can enumerate the set of initiators then it becomes clear
> whether sysfs can answer a performance enumeration question or if the
> application needs to consult an interface with specific knowledge of a
> given initiator-target pairing.
> 
> It seems a precursor to these patches is arranges for offline node
> devices to be created for the ACPI proximity domains that are
> offline-by default for reserved memory ranges.

The intention is that all initiators symlinked to the memory node share
the initiator_access attributes, as well as itself the node is its own
initiator. There's no limit to how many the new kernel interface in
patch 1/7 allows you to register, so it's not really a 1:1 relationship.

Either instead or in addition to the symlinks, we can export a node_mask
in the initiator_access directory for which these access attributes
apply if that makes the intention more clear.