Re: [RFC PATCH v4 1/7] mm/demotion: Add support for explicit memory tiers

[Aneesh Kumar K.V]

Aneesh Kumar K V <aneesh.kumar@xxxxxxxxxxxxx> writes:

> On 6/6/22 11:03 AM, Ying Huang wrote:
>> On Mon, 2022-06-06 at 09:26 +0530, Aneesh Kumar K V wrote:
>>> On 6/6/22 8:19 AM, Ying Huang wrote:
>>>> On Thu, 2022-06-02 at 14:07 +0800, Ying Huang wrote:
>>>>> On Fri, 2022-05-27 at 17:55 +0530, Aneesh Kumar K.V wrote:
>>>>>> From: Jagdish Gediya <jvgediya@xxxxxxxxxxxxx>
>>>>>>
>>>>>> In the current kernel, memory tiers are defined implicitly via a
>>>>>> demotion path relationship between NUMA nodes, which is created
>>>>>> during the kernel initialization and updated when a NUMA node is
>>>>>> hot-added or hot-removed.  The current implementation puts all
>>>>>> nodes with CPU into the top tier, and builds the tier hierarchy
>>>>>> tier-by-tier by establishing the per-node demotion targets based
>>>>>> on the distances between nodes.
>>>>>>
>>>>>> This current memory tier kernel interface needs to be improved for
>>>>>> several important use cases,
>>>>>>
>>>>>> The current tier initialization code always initializes
>>>>>> each memory-only NUMA node into a lower tier.  But a memory-only
>>>>>> NUMA node may have a high performance memory device (e.g. a DRAM
>>>>>> device attached via CXL.mem or a DRAM-backed memory-only node on
>>>>>> a virtual machine) and should be put into a higher tier.
>>>>>>
>>>>>> The current tier hierarchy always puts CPU nodes into the top
>>>>>> tier. But on a system with HBM or GPU devices, the
>>>>>> memory-only NUMA nodes mapping these devices should be in the
>>>>>> top tier, and DRAM nodes with CPUs are better to be placed into the
>>>>>> next lower tier.
>>>>>>
>>>>>> With current kernel higher tier node can only be demoted to selected nodes on the
>>>>>> next lower tier as defined by the demotion path, not any other
>>>>>> node from any lower tier.  This strict, hard-coded demotion order
>>>>>> does not work in all use cases (e.g. some use cases may want to
>>>>>> allow cross-socket demotion to another node in the same demotion
>>>>>> tier as a fallback when the preferred demotion node is out of
>>>>>> space), This demotion order is also inconsistent with the page
>>>>>> allocation fallback order when all the nodes in a higher tier are
>>>>>> out of space: The page allocation can fall back to any node from
>>>>>> any lower tier, whereas the demotion order doesn't allow that.
>>>>>>
>>>>>> The current kernel also don't provide any interfaces for the
>>>>>> userspace to learn about the memory tier hierarchy in order to
>>>>>> optimize its memory allocations.
>>>>>>
>>>>>> This patch series address the above by defining memory tiers explicitly.
>>>>>>
>>>>>> This patch adds below sysfs interface which is read-only and
>>>>>> can be used to read nodes available in specific tier.
>>>>>>
>>>>>> /sys/devices/system/memtier/memtierN/nodelist
>>>>>>
>>>>>> Tier 0 is the highest tier, while tier MAX_MEMORY_TIERS - 1 is the
>>>>>> lowest tier. The absolute value of a tier id number has no specific
>>>>>> meaning. what matters is the relative order of the tier id numbers.
>>>>>>
>>>>>> All the tiered memory code is guarded by CONFIG_TIERED_MEMORY.
>>>>>> Default number of memory tiers are MAX_MEMORY_TIERS(3). All the
>>>>>> nodes are by default assigned to DEFAULT_MEMORY_TIER(1).
>>>>>>
>>>>>> Default memory tier can be read from,
>>>>>> /sys/devices/system/memtier/default_tier
>>>>>>
>>>>>> Max memory tier can be read from,
>>>>>> /sys/devices/system/memtier/max_tiers
>>>>>>
>>>>>> This patch implements the RFC spec sent by Wei Xu <weixugc@xxxxxxxxxx> at [1].
>>>>>>
>>>>>> [1] https://lore.kernel.org/linux-mm/CAAPL-u-DGLcKRVDnChN9ZhxPkfxQvz9Sb93kVoX_4J2oiJSkUw@xxxxxxxxxxxxxx/
>>>>>>
>>>>>> Signed-off-by: Jagdish Gediya <jvgediya@xxxxxxxxxxxxx>
>>>>>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
>>>>>
>>>>> IMHO, we should change the kernel internal implementation firstly, then
>>>>> implement the kerne/user space interface.  That is, make memory tier
>>>>> explicit inside kernel, then expose it to user space.
>>>>
>>>> Why ignore this comment for v5?  If you don't agree, please respond me.
>>>>
>>>
>>> I am not sure what benefit such a rearrange would bring in? Right now I
>>> am writing the series from the point of view of introducing all the
>>> plumbing and them switching the existing demotion logic to use the new
>>> infrastructure. Redoing the code to hide all the userspace sysfs till we
>>> switch the demotion logic to use the new infrastructure doesn't really
>>> bring any additional clarity to patch review and would require me to
>>> redo the series with a lot of conflicts across the patches in the patchset.
>> 
>> IMHO, we shouldn't introduce regression even in the middle of a
>> patchset.  Each step should only rely on previous patches in the series
>> to work correctly.  In your current way of organization, after patch
>> [1/7], on a system with 2 memory tiers, the user space interface will
>> output wrong information (only 1 memory tier).  So I think the correct
>> way is to make it right inside the kenrel firstly, then expose the right
>> information to user space.
>>
>
> The patchset doesn't add additional tier until "mm/demotion/dax/kmem: 
> Set node's memory tier to MEMORY_TIER_PMEM". ie, there is no additional 
> tiers done till all the demotion logic is in place. So even if the 
> system got dax/kmem, the support for adding dax/kmem as a memory tier 
> comes later in the patch series.

Let me clarify this a bit more. This patchset doesn't change the
existing kernel behavior till "mm/demotion: Build demotion targets
based on explicit memory tiers". So there is no regression till then.
It adds a parallel framework (memory tiers to the existing demotion
logic).

I can move the patch "mm/demotion/dax/kmem: Set node's memory tier to
MEMORY_TIER_PMEM" before switching the demotion logic so that on systems
with two memory tiers (DRAM and pmem) the demotion continues to work
as expected after patch 3 ("mm/demotion: Build demotion targets based on
explicit memory tiers"). With that, there will not be any regression in
between the patch series.

-aneesh