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

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Message-ID: <f759cc28-309d-930c-da7d-34144a4d5517@deltatee.com>
Date:   Tue, 4 Dec 2018 12:41:39 -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>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        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 2018-12-04 12:22 p.m., Jerome Glisse wrote:
> So version is a bad prefix, what about type, prefixing target with a
> type id. So that application that are looking for a certain type of
> memory (which has a set of define properties) can select them. Having
> a type file inside the directory and hopping application will read
> that sysfs file is a recipies for failure from my point of view. While
> having it in the directory name is making sure that the application
> has some idea of what it is doing.

Well I don't think it can be a prefix. It has to be a mask. It might be
things like cache coherency, persistence, bandwidth and none of those
things are mutually exclusive.

>> Also, in the same vein, I think it's wrong to have the API enumerate all
>> the different memory available in the system. The API should simply
>> allow userspace to say it wants memory that can be accessed by a set of
>> initiators with a certain set of attributes and the bind call tries to
>> fulfill that or fallback on system memory/hmm migration/whatever.
> 
> We have existing application that use topology today to partition their
> workload and do load balancing. Those application leverage the fact that
> they are only running on a small set of known platform with known topology
> here i want to provide a common API so that topology can be queried in a
> standard by application.

Existing applications are not a valid excuse for poor API design.
Remember, once this API is introduced and has real users, it has to be
maintained *forever*, so we need to get it right. Providing users with
more information than they need makes it exponentially harder to get
right and support.

Logan