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Date:   Thu, 16 Jun 2022 10:17:40 +0530
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Subject: Re: [PATCH v5 1/9] mm/demotion: Add support for explicit memory tiers
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To:     Wei Xu <weixugc@google.com>, Ying Huang <ying.huang@intel.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        Linux MM <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Greg Thelen <gthelen@google.com>,
        Yang Shi <shy828301@gmail.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Tim C Chen <tim.c.chen@intel.com>,
        Brice Goglin <brice.goglin@gmail.com>,
        Michal Hocko <mhocko@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Hesham Almatary <hesham.almatary@huawei.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Alistair Popple <apopple@nvidia.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Feng Tang <feng.tang@intel.com>,
        Jagdish Gediya <jvgediya@linux.ibm.com>,
        Baolin Wang <baolin.wang@linux.alibaba.com>,
        David Rientjes <rientjes@google.com>
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From:   Aneesh Kumar K V <aneesh.kumar@linux.ibm.com>
In-Reply-To: <CAAPL-u9hjR8sFeUYM46TXPg-Lho6hr0zaLvrGhg9=htRrPtjqw@mail.gmail.com>
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On 6/16/22 9:15 AM, Wei Xu wrote:
> On Wed, Jun 15, 2022 at 6:11 PM Ying Huang <ying.huang@intel.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