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SubjectRe: [PATCH v7 11/12] mm/demotion: Add documentation for memory tiering
On Wed, Jun 22, 2022 at 01:55:12PM +0530, Aneesh Kumar K.V wrote:
> From: Jagdish Gediya <jvgediya@linux.ibm.com>
>

Hi Aneesh and Jagdish,

The documentation can be improved, see below.

> All N_MEMORY nodes are divided into 3 memoty tiers with tier ID value
> MEMORY_TIER_HBM_GPU, MEMORY_TIER_DRAM and MEMORY_TIER_PMEM. By default,
> all nodes are assigned to default memory tier.
>
> Demotion path for all N_MEMORY nodes is prepared based on the tier ID value
> of memory tiers.
>
> This patch adds documention for memory tiering introduction, its sysfs
> interfaces and how demotion is performed based on memory tiers.
>

I think the patch message should just be:
"Add documentation for memory tiering. It also covers its sysfs
interfaces and how demotion is performed based on memory tiers."

> +===========
> +Memory tiers
> +============
> +
> +This document describes explicit memory tiering support along with
> +demotion based on memory tiers.
> +

This causes htmldocs error, for which I have applied the fixup at [1].

> +Memory nodes are divided into 3 types of memory tiers with tier ID
> +value as shown based on their hardware characteristics.
> +
> +
> +MEMORY_TIER_HBM_GPU
> +MEMORY_TIER_DRAM
> +MEMORY_TIER_PMEM
> +

Use bullet list.

> +Sysfs interfaces
> +================
> +
> +Nodes belonging to specific tier can be read from,
> +/sys/devices/system/memtier/memtierN/nodelist (Read-Only)
> +
> +Where N is 0 - 2.

The "where" sentence can be compounded into the previous sentence above.

> +
> +Example 1:
> +For a system where Node 0 is CPU + DRAM nodes, Node 1 is HBM node,
> +node 2 is a PMEM node an ideal tier layout will be
> +
> +$ cat /sys/devices/system/memtier/memtier0/nodelist
> +1
> +$ cat /sys/devices/system/memtier/memtier1/nodelist
> +0
> +$ cat /sys/devices/system/memtier/memtier2/nodelist
> +2
> +

The code snippets should have been inside literal code blocks.

> +Example 2:
> +For a system where Node 0 & 1 are CPU + DRAM nodes, node 2 & 3 are PMEM
> +nodes.
> +
> +$ cat /sys/devices/system/memtier/memtier0/nodelist
> +cat: /sys/devices/system/memtier/memtier0/nodelist: No such file or
> +directory
> +$ cat /sys/devices/system/memtier/memtier1/nodelist
> +0-1
> +$ cat /sys/devices/system/memtier/memtier2/nodelist
> +2-3
> +

Use literal code block.

> +Default memory tier can be read from,
> +/sys/devices/system/memtier/default_tier (Read-Only)
> +
> +e.g.
> +$ cat /sys/devices/system/memtier/default_tier
> +memtier200
> +
> +Max memory tier ID supported can be read from,
> +/sys/devices/system/memtier/max_tier (Read-Only)
> +
> +e.g.
> +$ cat /sys/devices/system/memtier/max_tier
> +400
> +
> +Individual node's memory tier can be read of set using,
> +/sys/devices/system/node/nodeN/memtier (Read-Write)
> +
> +where N = node id
> +
> +When this interface is written, Node is moved from the old memory tier
> +to new memory tier and demotion targets for all N_MEMORY nodes are
> +built again.
> +
> +For example 1 mentioned above,
> +$ cat /sys/devices/system/node/node0/memtier
> +1
> +$ cat /sys/devices/system/node/node1/memtier
> +0
> +$ cat /sys/devices/system/node/node2/memtier
> +2
> +

The same suggestions above apply here, too.

> +Enable/Disable demotion
> +-----------------------
> +
> +By default demotion is disabled, it can be enabled/disabled using
> +below sysfs interface,
> +
> +$ echo 0/1 or false/true > /sys/kernel/mm/numa/demotion_enabled
> +

Use literal code block.

> +preferred and allowed demotion nodes
> +------------------------------------
> +
> +Preferred nodes for a specific N_MEMORY node are the best nodes
> +from the next possible lower memory tier. Allowed nodes for any
> +node are all the nodes available in all possible lower memory
> +tiers.
> +
> +Example:
> +
> +For a system where Node 0 & 1 are CPU + DRAM nodes, node 2 & 3 are PMEM
> +nodes,
> +
> +node distances:
> +node 0 1 2 3
> + 0 10 20 30 40
> + 1 20 10 40 30
> + 2 30 40 10 40
> + 3 40 30 40 10
> +

Use reST table.

> +memory_tiers[0] = <empty>
> +memory_tiers[1] = 0-1
> +memory_tiers[2] = 2-3
> +
> +node_demotion[0].preferred = 2
> +node_demotion[0].allowed = 2, 3
> +node_demotion[1].preferred = 3
> +node_demotion[1].allowed = 3, 2
> +node_demotion[2].preferred = <empty>
> +node_demotion[2].allowed = <empty>
> +node_demotion[3].preferred = <empty>
> +node_demotion[3].allowed = <empty>
> +

What are these above? Node properties? BTW, use literal code block.

If you don't understand these suggestions above, here is the diff:

---- >8 ----

diff --git a/Documentation/admin-guide/mm/memory-tiering.rst b/Documentation/admin-guide/mm/memory-tiering.rst
index 0a75e0dab1fd8e..10ec5aab6ddd53 100644
--- a/Documentation/admin-guide/mm/memory-tiering.rst
+++ b/Documentation/admin-guide/mm/memory-tiering.rst
@@ -14,13 +14,13 @@ Introduction

Many systems have multiple types of memory devices e.g. GPU, DRAM and
PMEM. The memory subsystem of these systems can be called a memory
-tiering system because the performance of the different types of
+tiering system because the performance of each type of
memory is different. Memory tiers are defined based on the hardware
capabilities of memory nodes. Each memory tier is assigned a tier ID
value that determines the memory tier position in demotion order.

The memory tier assignment of each node is independent of each
-other. Moving a node from one tier to another tier doesn't affect
+other. Moving a node from one tier to another doesn't affect
the tier assignment of any other node.

Memory tiers are used to build the demotion targets for nodes. A node
@@ -32,10 +32,9 @@ Memory tier rank
Memory nodes are divided into 3 types of memory tiers with tier ID
value as shown based on their hardware characteristics.

-
-MEMORY_TIER_HBM_GPU
-MEMORY_TIER_DRAM
-MEMORY_TIER_PMEM
+ * MEMORY_TIER_HBM_GPU
+ * MEMORY_TIER_DRAM
+ * MEMORY_TIER_PMEM

Memory tiers initialization and (re)assignments
===============================================
@@ -49,68 +48,73 @@ hotplug, the memory tier with default tier ID is assigned to the memory node.
Sysfs interfaces
================

-Nodes belonging to specific tier can be read from,
-/sys/devices/system/memtier/memtierN/nodelist (Read-Only)
+Nodes belonging to specific tier can be read from
+/sys/devices/system/memtier/memtierN/nodelist, where N is 0 - 2 (read-only)

-Where N is 0 - 2.
+Examples:

-Example 1:
-For a system where Node 0 is CPU + DRAM nodes, Node 1 is HBM node,
-node 2 is a PMEM node an ideal tier layout will be
+1. On a system where Node 0 is CPU + DRAM nodes, Node 1 is HBM node,
+ node 2 is a PMEM node an ideal tier layout will be:

-$ cat /sys/devices/system/memtier/memtier0/nodelist
-1
-$ cat /sys/devices/system/memtier/memtier1/nodelist
-0
-$ cat /sys/devices/system/memtier/memtier2/nodelist
-2
+ .. code-block::

-Example 2:
-For a system where Node 0 & 1 are CPU + DRAM nodes, node 2 & 3 are PMEM
-nodes.
+ $ cat /sys/devices/system/memtier/memtier0/nodelist
+ 1
+ $ cat /sys/devices/system/memtier/memtier1/nodelist
+ 0
+ $ cat /sys/devices/system/memtier/memtier2/nodelist
+ 2

-$ cat /sys/devices/system/memtier/memtier0/nodelist
-cat: /sys/devices/system/memtier/memtier0/nodelist: No such file or
-directory
-$ cat /sys/devices/system/memtier/memtier1/nodelist
-0-1
-$ cat /sys/devices/system/memtier/memtier2/nodelist
-2-3
+2. On a system where Node 0 & 1 are CPU + DRAM nodes, node 2 & 3 are PMEM
+ nodes:

-Default memory tier can be read from,
-/sys/devices/system/memtier/default_tier (Read-Only)
+ .. code-block::

-e.g.
-$ cat /sys/devices/system/memtier/default_tier
-memtier200
+ $ cat /sys/devices/system/memtier/memtier0/nodelist
+ cat: /sys/devices/system/memtier/memtier0/nodelist: No such file or
+ directory
+ $ cat /sys/devices/system/memtier/memtier1/nodelist
+ 0-1
+ $ cat /sys/devices/system/memtier/memtier2/nodelist
+ 2-3

-Max memory tier ID supported can be read from,
-/sys/devices/system/memtier/max_tier (Read-Only)
+Default memory tier can be read from
+/sys/devices/system/memtier/default_tier (read-only), e.g.:

-e.g.
-$ cat /sys/devices/system/memtier/max_tier
-400
+.. code-block::

-Individual node's memory tier can be read of set using,
-/sys/devices/system/node/nodeN/memtier (Read-Write)
+ $ cat /sys/devices/system/memtier/default_tier
+ memtier200

-where N = node id
+Max memory tier ID supported can be read from
+/sys/devices/system/memtier/max_tier (read-only), e.g.:

-When this interface is written, Node is moved from the old memory tier
+.. code-block::
+
+ $ cat /sys/devices/system/memtier/max_tier
+ 400
+
+Individual node's memory tier can be read or set using
+/sys/devices/system/node/nodeN/memtier (read-write), where N = node id.
+
+When this interface is written, node is moved from the old memory tier
to new memory tier and demotion targets for all N_MEMORY nodes are
built again.

-For example 1 mentioned above,
-$ cat /sys/devices/system/node/node0/memtier
-1
-$ cat /sys/devices/system/node/node1/memtier
-0
-$ cat /sys/devices/system/node/node2/memtier
-2
+For example 1 mentioned above:
+
+.. code-block::
+
+ $ cat /sys/devices/system/node/node0/memtier
+ 1
+ $ cat /sys/devices/system/node/node1/memtier
+ 0
+ $ cat /sys/devices/system/node/node2/memtier
+ 2

Additional memory tiers can be created by writing a tier ID value to this file.
-This results in a new memory tier creation and moving the specific NUMA node to
-that memory tier.
+This results into creating a new tier and moving the specific NUMA node to
+that tier.

Demotion
========
@@ -128,19 +132,20 @@ be used.

Instead of a page being discarded during reclaim, it can be moved to
persistent memory. Allowing page migration during reclaim enables
-these systems to migrate pages from fast(higher) tiers to slow(lower)
-tiers when the fast(higher) tier is under pressure.
+these systems to migrate pages from fast (higher) tiers to slow (lower)
+tiers when the fast (higher) tier is under pressure.


Enable/Disable demotion
-----------------------

-By default demotion is disabled, it can be enabled/disabled using
-below sysfs interface,
+By default demotion is disabled. It can be toggled by:

-$ echo 0/1 or false/true > /sys/kernel/mm/numa/demotion_enabled
+.. code-block::

-preferred and allowed demotion nodes
+ $ echo 0/1 or false/true > /sys/kernel/mm/numa/demotion_enabled
+
+Preferred and allowed demotion nodes
------------------------------------

Preferred nodes for a specific N_MEMORY node are the best nodes
@@ -148,35 +153,40 @@ from the next possible lower memory tier. Allowed nodes for any
node are all the nodes available in all possible lower memory
tiers.

-Example:
+For example, on a system where Node 0 & 1 are CPU + DRAM nodes,
+node 2 & 3 are PMEM nodes:

-For a system where Node 0 & 1 are CPU + DRAM nodes, node 2 & 3 are PMEM
-nodes,
+ * node distances

-node distances:
-node 0 1 2 3
- 0 10 20 30 40
- 1 20 10 40 30
- 2 30 40 10 40
- 3 40 30 40 10
+ ==== == == == ==
+ node 0 1 2 3
+ ==== == == == ==
+ 0 10 20 30 40
+ 1 20 10 40 30
+ 2 30 40 10 40
+ 3 40 30 40 10
+ ==== == == == ==

-memory_tiers[0] = <empty>
-memory_tiers[1] = 0-1
-memory_tiers[2] = 2-3
+ * node properties

-node_demotion[0].preferred = 2
-node_demotion[0].allowed = 2, 3
-node_demotion[1].preferred = 3
-node_demotion[1].allowed = 3, 2
-node_demotion[2].preferred = <empty>
-node_demotion[2].allowed = <empty>
-node_demotion[3].preferred = <empty>
-node_demotion[3].allowed = <empty>
+ .. code-block::
+
+ memory_tiers[0] = <empty>
+ memory_tiers[1] = 0-1
+ memory_tiers[2] = 2-3
+
+ node_demotion[0].preferred = 2
+ node_demotion[0].allowed = 2, 3
+ node_demotion[1].preferred = 3
+ node_demotion[1].allowed = 3, 2
+ node_demotion[2].preferred = <empty>
+ node_demotion[2].allowed = <empty>
+ node_demotion[3].preferred = <empty>
+ node_demotion[3].allowed = <empty>

Memory allocation for demotion
------------------------------

-If a page needs to be demoted from any node, the kernel 1st tries
-to allocate a new page from the node's preferred node and fallbacks to
-node's allowed targets in allocation fallback order.
-
+If a page needs to be demoted from any node, the kernel first tries
+to allocate a new page from the node's preferred target node and fallbacks
+to node's allowed targets in allocation fallback order.

Thanks.

[1]: https://lore.kernel.org/linux-doc/YrZ5cTFOSuWxlF2t@debian.me/

--
An old man doll... just what I always wanted! - Clara

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