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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Tue, 27 Nov 2018 09:42:54 -0800
Message-ID: <CAPcyv4hMFc7K=FjHWiMVAiOxVC-s0itPjVTs_-7KrFhg4h_SXQ@mail.gmail.com>
Subject: Re: [PATCH 2/7] node: Add heterogenous memory performance
To:     Keith Busch <keith.busch@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>,
        Dave Hansen <dave.hansen@intel.com>
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On Mon, Nov 26, 2018 at 11:00 PM Dan Williams <dan.j.williams@intel.com> 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.

Sorry, I misread patch1, this series does allow publishing the
multi-initiator case that shares the same performance profile to a
given target.

> 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.

Likely still need this though because node devices don't tend to show
up until they have a cpu or online memory.