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Date:   Tue, 5 Dec 2023 10:09:16 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Srinivasulu Thanneeru <sthanneeru.opensrc@micron.com>
Cc:     aneesh.kumar@linux.ibm.com, linux-cxl@vger.kernel.org,
        linux-mm@kvack.org, dan.j.williams@intel.com, hannes@cmpxchg.org,
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        hezhongkun.hzk@bytedance.com, fvdl@google.com, john@jagalactic.com,
        emirakhur@micron.com, vtavarespetr@micron.com,
        Ravis.OpenSrc@micron.com, Jonathan.Cameron@huawei.com,
        linux-kernel@vger.kernel.org, linux-api@vger.kernel.org
Subject: Re: [EXT] Re: [RFC PATCH 0/2] Node migration between memory tiers
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On Tue 05-12-23 14:32:20, Srinivasulu Thanneeru wrote:
> 
> 
> On 12/5/2023 2:21 PM, Michal Hocko wrote:
> > CAUTION: EXTERNAL EMAIL. Do not click links or open attachments unless you recognize the sender and were expecting this message.
> > 
> > 
> > On Tue 05-12-23 14:12:17, Srinivasulu Thanneeru wrote:
> > > 
> > > 
> > > On 12/5/2023 2:05 PM, Michal Hocko wrote:
> > > > CAUTION: EXTERNAL EMAIL. Do not click links or open attachments unless you recognize the sender and were expecting this message.
> > > > 
> > > > 
> > > > On Tue 05-12-23 01:26:07, Srinivasulu Thanneeru wrote:
> > > > > 
> > > > > 
> > > > > On 12/4/2023 9:13 PM, Michal Hocko wrote:
> > > > > > CAUTION: EXTERNAL EMAIL. Do not click links or open attachments unless you recognize the sender and were expecting this message.
> > > > > > 
> > > > > > 
> > > > > > On Fri 01-12-23 03:34:20, sthanneeru.opensrc@micron.com wrote:
> > > > > > > From: Srinivasulu Thanneeru <sthanneeru.opensrc@micron.com>
> > > > > > > 
> > > > > > > The memory tiers feature allows nodes with similar memory types
> > > > > > > or performance characteristics to be grouped together in a
> > > > > > > memory tier. However, there is currently no provision for
> > > > > > > moving a node from one tier to another on demand.
> > > > > > 
> > > > > > Could you expand on why this is really needed/necessary? What is the
> > > > > > actual usecase?
> > > > > 
> > > > > Hi Michal Hock,
> > > > > 
> > > > > Following two use-cases we have observed.
> > > > > 1. It is not accurate to group similar memory types in the same tier,
> > > > >      because even similar memory types may have different speed grades.
> > > > 
> > > > Presumably they are grouped based on a HW configuration. Does that mean
> > > > that the configuration is wrong? Are you trying to workaround that by
> > > > this interface?
> > > > 
> > > > > 2. Some systems boots up with CXL devices and DRAM on the same memory-tier,
> > > > > we need a way to move the CXL nodes to the correct tier from the user space.
> > > > 
> > > > Again, could you expand a bit more and explain why this cannot be
> > > > configured automatically?
> > > 
> > > Yes, in both cases above, if hardware not automatically populated properly,
> > > in that case this interface would help to correct it from user space.
> > > 
> > > We had observed case-2 in our setups.
> > 
> > How hard it is to address this at the HW level?
> > 
> > Btw. this is really important piece of context that should be part of
> > the changelog. Quite honestly introducing user interfaces solely to
> > workaround HW issues seems a rather weak justification. Are there any
> > usecases you can think of where this would be useful?
> 
> I'm not sure how difficult to fix it in the hardware.

Please explore that. It is sad to see learn that CXL which is a really
new technology is already fighting with misconfigurations.

-- 
Michal Hocko
SUSE Labs