linux-kernel - Re: [PATCH v5 1/9] mm/demotion: Add support for explicit memory tiers

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Date:   Thu, 16 Jun 2022 10:17:40 +0530
From:   Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>
To:     Wei Xu <weixugc@...gle.com>, Ying Huang <ying.huang@...el.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Thelen <gthelen@...gle.com>,
        Yang Shi <shy828301@...il.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim C Chen <tim.c.chen@...el.com>,
        Brice Goglin <brice.goglin@...il.com>,
        Michal Hocko <mhocko@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Hesham Almatary <hesham.almatary@...wei.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Alistair Popple <apopple@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Feng Tang <feng.tang@...el.com>,
        Jagdish Gediya <jvgediya@...ux.ibm.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v5 1/9] mm/demotion: Add support for explicit memory tiers

On 6/16/22 9:15 AM, Wei Xu wrote:
> On Wed, Jun 15, 2022 at 6:11 PM Ying Huang <ying.huang@...el.com> wrote:
>>
>> On Tue, 2022-06-14 at 14:56 -0400, Johannes Weiner wrote:
>>> On Tue, Jun 14, 2022 at 01:31:37PM +0530, Aneesh Kumar K V wrote:

....

>> As Jonathan Cameron pointed, we may need the memory tier ID to be
>> stable if possible.  I know this isn't a easy task.  At least we can
>> make the default memory tier (CPU local DRAM) ID stable (for example
>> make it always 128)?  That provides an anchor for users to understand.
> 
> One of the motivations of introducing "rank" is to allow memory tier
> ID to be stable, at least for the well-defined tiers such as the
> default memory tier.  The default memory tier can be moved around in
> the tier hierarchy by adjusting its rank position relative to other
> tiers, but its device ID can remain the same, e.g. always 1.
> 

With /sys/devices/system/memtier/default_tier userspace will be able query
the default tier details. Did you get to look at 

https://lore.kernel.org/linux-mm/7b72ccf4-f4ae-cb4e-f411-74d055482026@linux.ibm.com

Any reason why that will not work with all the requirements we had?

-aneesh