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Date: Mon, 18 Dec 2023 08:56:02 +0000
From: Srinivasulu Thanneeru <sthanneeru@...ron.com>
To: "Huang, Ying" <ying.huang@...el.com>, Srinivasulu Opensrc
	<sthanneeru.opensrc@...ron.com>
CC: "linux-cxl@...r.kernel.org" <linux-cxl@...r.kernel.org>,
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Subject: Re: [EXT] Re: [RFC PATCH v2 0/2] Node migration between memory tiers


Micron Confidential



Micron Confidential
________________________________________
From: Huang, Ying <ying.huang@...el.com>
Sent: Friday, December 15, 2023 10:32 AM
To: Srinivasulu Opensrc
Cc: linux-cxl@...r.kernel.org; linux-mm@...ck.org; Srinivasulu Thanneeru; aneesh.kumar@...ux.ibm.com; dan.j.williams@...el.com; gregory.price; mhocko@...e.com; tj@...nel.org; john@...alactic.com; Eishan Mirakhur; Vinicius Tavares Petrucci; Ravis OpenSrc; Jonathan.Cameron@...wei.com; linux-kernel@...r.kernel.org
Subject: [EXT] Re: [RFC PATCH v2 0/2] Node migration between memory tiers

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<sthanneeru.opensrc@...ron.com> writes:

> From: Srinivasulu Thanneeru <sthanneeru.opensrc@...ron.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.
>
> This patch series aims to support node migration between tiers
> on demand by sysadmin/root user using the provided sysfs for
> node migration.
>
> To migrate a node to a tier, the corresponding node’s sysfs
> memtier_override is written with target tier id.
>
> Example: Move node2 to memory tier2 from its default tier(i.e 4)
>
> 1. To check current memtier of node2
> $cat  /sys/devices/system/node/node2/memtier_override
> memory_tier4
>
> 2. To migrate node2 to memory_tier2
> $echo 2 > /sys/devices/system/node/node2/memtier_override
> $cat  /sys/devices/system/node/node2/memtier_override
> memory_tier2
>
> Usecases:
>
> 1. Useful to move cxl nodes to the right tiers from userspace, when
>    the hardware fails to assign the tiers correctly based on
>    memorytypes.
>
>    On some platforms we have observed cxl memory being assigned to
>    the same tier as DDR memory. This is arguably a system firmware
>    bug, but it is true that tiers represent *ranges* of performance
>    and we believe it's important for the system operator to have
>    the ability to override bad firmware or OS decisions about tier
>    assignment as a fail-safe against potential bad outcomes.
>
> 2. Useful if we want interleave weights to be applied on memory tiers
>    instead of nodes.
> In a previous thread, Huang Ying <ying.huang@...el.com> thought
> this feature might be useful to overcome limitations of systems
> where nodes with different bandwidth characteristics are grouped
> in a single tier.
> https://lore.kernel.org/lkml/87a5rw1wu8.fsf@yhuang6-desk2.ccr.corp.intel.com/
>
> =============
> Version Notes:
>
> V2 : Changed interface to memtier_override from adistance_offset.
> memtier_override was recommended by
> 1. John Groves <john@...alactic.com>
> 2. Ravi Shankar <ravis.opensrc@...ron.com>
> 3. Brice Goglin <Brice.Goglin@...ia.fr>

It appears that you ignored my comments for V1 as follows ...

https://lore.kernel.org/lkml/87o7f62vur.fsf@yhuang6-desk2.ccr.corp.intel.com/

Thank you Huang, Ying for pointing to this.

https://lpc.events/event/16/contributions/1209/attachments/1042/1995/Live%20In%20a%20World%20With%20Multiple%20Memory%20Types.pdf

In the presentation above, the adistance_offsets are per memtype.
We believe that adistance_offset per node is more suitable and flexible
since we can change it per node. If we keep adistance_offset per memtype,
then we cannot change it for a specific node of a given memtype.


https://lore.kernel.org/lkml/87jzpt2ft5.fsf@yhuang6-desk2.ccr.corp.intel.com/

I guess that you need to move all NUMA nodes with same performance
metrics together?  If so, That is why we previously proposed to place
the knob in "memory_type"? (From: Huang, Ying )

Yes, memory_type would be group the related memories togather as single tier.
We should also have a flexibility to move nodes between tiers, to address the issues described in usecases above.

https://lore.kernel.org/lkml/87a5qp2et0.fsf@yhuang6-desk2.ccr.corp.intel.com/

This patch provides a way to move a node to the correct tier.
We observed in test setups where DRAM and CXL are put under the same
tier (memory_tier4).
By using this patch, we can move the CXL node away from the DRAM-linked
tier4 and put it in the desired tier.

Regards,
Srini

--
Best Regards,
Huang, Ying

> V1 : Introduced adistance_offset sysfs.
>
> =============
>
> Srinivasulu Thanneeru (2):
>   base/node: Add sysfs for memtier_override
>   memory tier: Support node migration between tiers
>
>  Documentation/ABI/stable/sysfs-devices-node |  7 ++
>  drivers/base/node.c                         | 47 ++++++++++++
>  include/linux/memory-tiers.h                | 11 +++
>  include/linux/node.h                        | 11 +++
>  mm/memory-tiers.c                           | 85 ++++++++++++---------
>  5 files changed, 125 insertions(+), 36 deletions(-)

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