Re: [PATCH v5 1/9] mm/demotion: Add support for explicit memory tiers

[Aneesh Kumar K V]

On 6/8/22 3:02 AM, Yang Shi wrote:
> On Fri, Jun 3, 2022 at 6:43 AM Aneesh Kumar K.V
> <aneesh.kumar@xxxxxxxxxxxxx> wrote:
> > 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 introduce explicity memory tiers with ranks. The rank
> > value of a memory tier is used to derive the demotion order between
> > NUMA nodes. The memory tiers present in a system can be found at
> >
> > /sys/devices/system/memtier/memtierN/
> >
> > The nodes which are part of a specific memory tier can be listed
> > via
> > /sys/devices/system/memtier/memtierN/nodelist
> >
> > "Rank" is an opaque value. Its absolute value doesn't have any
> > special meaning. But the rank values of different memtiers can be
> > compared with each other to determine the memory tier order.
> >
> > For example, if we have 3 memtiers: memtier0, memtier1, memiter2, and
> > their rank values are 300, 200, 100, then the memory tier order is:
> > memtier0 -> memtier2 -> memtier1, where memtier0 is the highest tier
> > and memtier1 is the lowest tier.
> >
> > The rank value of each memtier should be unique.
> >
> > A higher rank memory tier will appear first in the demotion order
> > than a lower rank memory tier. ie. while reclaim we choose a node
> > in higher rank memory tier to demote pages to as compared to a node
> > in a lower rank memory tier.
> >
> > For now we are not adding the dynamic number of memory tiers.
> > But a future series supporting that is possible. Currently
> > number of tiers supported is limitted to MAX_MEMORY_TIERS(3).
> > When doing memory hotplug, if not added to a memory tier, the NUMA
> > node gets added to DEFAULT_MEMORY_TIER(1).
> >
> > This patch is based on the proposal sent by Wei Xu <weixugc@xxxxxxxxxx> at [1].
> >
> > [1] https://lore.kernel.org/linux-mm/CAAPL-u9Wv+nH1VOZTj=9p9S70Y3Qz3+63EkqncRDdHfubsrjfw@xxxxxxxxxxxxxx
> >
> > Suggested-by: Wei Xu <weixugc@xxxxxxxxxx>
> > Signed-off-by: Jagdish Gediya <jvgediya@xxxxxxxxxxxxx>
> > Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
> > ---
> >   include/linux/memory-tiers.h |  20 ++++
> >   mm/Kconfig                   |  11 ++
> >   mm/Makefile                  |   1 +
> >   mm/memory-tiers.c            | 188 +++++++++++++++++++++++++++++++++++
> >   4 files changed, 220 insertions(+)
> >   create mode 100644 include/linux/memory-tiers.h
> >   create mode 100644 mm/memory-tiers.c
> >
> > diff --git a/include/linux/memory-tiers.h b/include/linux/memory-tiers.h
> > new file mode 100644
> > index 000000000000..e17f6b4ee177
> > --- /dev/null
> > +++ b/include/linux/memory-tiers.h
> > @@ -0,0 +1,20 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +#ifndef _LINUX_MEMORY_TIERS_H
> > +#define _LINUX_MEMORY_TIERS_H
> > +
> > +#ifdef CONFIG_TIERED_MEMORY
> > +
> > +#define MEMORY_TIER_HBM_GPU    0
> > +#define MEMORY_TIER_DRAM       1
> > +#define MEMORY_TIER_PMEM       2
> > +
> > +#define MEMORY_RANK_HBM_GPU    300
> > +#define MEMORY_RANK_DRAM       200
> > +#define MEMORY_RANK_PMEM       100
> > +
> > +#define DEFAULT_MEMORY_TIER    MEMORY_TIER_DRAM
> > +#define MAX_MEMORY_TIERS  3
> > +
> > +#endif /* CONFIG_TIERED_MEMORY */
> > +
> > +#endif
> > diff --git a/mm/Kconfig b/mm/Kconfig
> > index 169e64192e48..08a3d330740b 100644
> > --- a/mm/Kconfig
> > +++ b/mm/Kconfig
> > @@ -614,6 +614,17 @@ config ARCH_ENABLE_HUGEPAGE_MIGRATION
> >   config ARCH_ENABLE_THP_MIGRATION
> >          bool
> >
> > +config TIERED_MEMORY
> > +       bool "Support for explicit memory tiers"
> > +       def_bool n
> > +       depends on MIGRATION && NUMA
> > +       help
> > +         Support to split nodes into memory tiers explicitly and
> > +         to demote pages on reclaim to lower tiers. This option
> > +         also exposes sysfs interface to read nodes available in
> > +         specific tier and to move specific node among different
> > +         possible tiers.
>
> IMHO we should not need a new kernel config. If tiering is not present
> then there is just one tier on the system. And tiering is a kind of
> hardware configuration, the information could be shown regardless of
> whether demotion/promotion is supported/enabled or not.

This was added so that we could avoid doing multiple

#if defined(CONFIG_MIGRATION) && defined(CONFIG_NUMA)

Initially I had that as def_bool y and depends on MIGRATION && NUMA. But 
it was later suggested that def_bool is not recommended for newer config.

How about

 config TIERED_MEMORY
 	bool "Support for explicit memory tiers"
-	def_bool n
-	depends on MIGRATION && NUMA
-	help
-	  Support to split nodes into memory tiers explicitly and
-	  to demote pages on reclaim to lower tiers. This option
-	  also exposes sysfs interface to read nodes available in
-	  specific tier and to move specific node among different
-	  possible tiers.
+	def_bool MIGRATION && NUMA

 config HUGETLB_PAGE_SIZE_VARIABLE
 	def_bool n

ie, we just make it a Kconfig variable without exposing it to the user?

-aneesh