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

[Aneesh Kumar K V]

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.

-aneesh