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

Aneesh Kumar K V
2022-06-06T06:02:17Z

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@linux.ibm.com>
>>>>>
>>>>> 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@google.com> at [1].
>>>>>
>>>>> [1] https://lore.kernel.org/linux-mm/CAAPL-u-DGLcKRVDnChN9ZhxPkfxQvz9Sb93kVoX_4J2oiJSkUw@mail.gmail.com/
>>>>>
>>>>> Signed-off-by: Jagdish Gediya <jvgediya@linux.ibm.com>
>>>>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
>>>>
>>>> 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.

-aneesh