linux-kernel - Re: [RFC PATCH 02/14] mm/hms: heterogenenous memory system (HMS) documentation

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Message-ID: <a0240f08-68ab-5167-c2c7-2f930aa0a54b@deltatee.com>
Date:   Wed, 5 Dec 2018 16:09:29 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Andi Kleen <ak@...ux.intel.com>, Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Haggai Eran <haggaie@...lanox.com>, balbirs@....ibm.com,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        "Kuehling, Felix" <felix.kuehling@....com>, Philip.Yang@....com,
        "Koenig, Christian" <christian.koenig@....com>,
        "Blinzer, Paul" <Paul.Blinzer@....com>,
        John Hubbard <jhubbard@...dia.com>, rcampbell@...dia.com
Subject: Re: [RFC PATCH 02/14] mm/hms: heterogenenous memory system (HMS)
 documentation



On 2018-12-05 3:58 p.m., Jerome Glisse wrote:
> So just to be clear here is how i understand your position:
> "Single coherent sysfs hierarchy to describe something is useless
>  let's git rm drivers/base/"

I have no idea what you're talking about. I'm saying the existing sysfs
hierarchy *should* be used for this application -- we shouldn't be
creating another hierarchy.

> While i am arguing that "hey the /sys/bus/node/devices/* is nice
> but it just does not cut it for all this new hardware platform
> if i add new nodes there for my new memory i will break tons of
> existing application. So what about a new hierarchy that allow
> to describe those new hardware platform in a single place like
> today node thing"

I'm talking about /sys/bus and all the bus information under there; not
just the node hierarchy. With this information, you can figure out how
any struct device is connected to another struct device. This has little
to do with a hypothetical memory device and what it might expose. You're
conflating memory devices with links between devices (ie. buses).


> No can do that is what i am trying to explain. So if i bus 1 in a
> sub-system A and usualy that kind of bus can serve a bridge for
> PCIE ie a CPU can access device behind it by going through a PCIE
> device first. So now the userspace libary have this knowledge
> bake in. Now if a platform has a bug for whatever reasons where
> that does not hold, the kernel has no way to tell userspace that
> there is an exception there. It is up to userspace to have a data
> base of quirks.

> Kernel see all those objects in isolation in your scheme. While in
> what i am proposing there is only one place and any device that
> participate in this common place can report any quirks so that a
> coherent view is given to user space.

The above makes no sense to me.


> If we have gazillion of places where all this informations is spread
> around than we have no way to fix weird inter-action between any
> of those.

So work to standardize it so that all buses present a consistent view of
what guarantees they provide for bus accesses. Quirks could then adjust
that information for systems that may be broken.

Logan