[v6,9/9] mm: multigenerational lru: Kconfig

Commit Message

Yu Zhao Jan. 4, 2022, 8:22 p.m. UTC

Add configuration options for the multigenerational lru.

Signed-off-by: Yu Zhao <[e-mail]>
Tested-by: Konstantin Kharlamov <[e-mail]>
---
 Documentation/vm/multigen_lru.rst | 18 ++++++++++++
 mm/Kconfig                        | 48 +++++++++++++++++++++++++++++++
 2 files changed, 66 insertions(+)

Comments

Linus Torvalds Jan. 4, 2022, 9:39 p.m. UTC | #1

On Tue, Jan 4, 2022 at 12:23 PM Yu Zhao <[e-mail]> wrote:
>
> Add configuration options for the multigenerational lru.
>         def_bool ARCH_HAS_SET_DIRECT_MAP && !EMBEDDED
>
> +config NR_LRU_GENS
> +       int "Max number of generations"
> +       depends on LRU_GEN
> +       range 4 31
> +       default 4
> +       help
> +         This option uses order_base_2(N+1) bits in page flags.
> +
> +         Do not configure more generations than you plan to use. They have a
> +         per-memcg and per-node memory overhead.
> +
> +config TIERS_PER_GEN
> +       int "Number of tiers per generation"
> +       depends on LRU_GEN
> +       range 2 5
> +       default 4
> +       help
> +         This option uses N-2 bits in page flags.
> +
> +         Larger values generally provide better protection for page cache when
> +         under heavy buffered I/O workloads.

These are not appropriate questions to ask users.

No user has any idea what the answer should be. And no, we don't add
"benchmark tuning Kconfig questions" to the kernel. We leave those
kinds of games to companies that need to fake their benchmark numbers.

If *you* can't give a good number for these config options, then no
user or distro can either.

So just pick a number, and stand by it.

Don't do this kind of "I don't know what the right number is, so I'll
just push the blame on the user".

                Linus

Patch

diff --git a/Documentation/vm/multigen_lru.rst b/Documentation/vm/multigen_lru.rst
index 6f9e0181348b..a54c5637c455 100644
--- a/Documentation/vm/multigen_lru.rst
+++ b/Documentation/vm/multigen_lru.rst
@@ -6,6 +6,13 @@  Multigenerational LRU
 
 Quick start
 ===========
+Build configurations
+--------------------
+:Required: Set ``CONFIG_LRU_GEN=y``.
+
+:Optional: Set ``CONFIG_LRU_GEN_ENABLED=y`` to enable this feature by
+ default.
+
 Runtime configurations
 ----------------------
 :Required: Write ``1`` to ``/sys/kernel/mm/lru_gen/enable`` if the
@@ -25,6 +32,17 @@  Personal computers
 
 Data centers
 ------------
+:Optional: Change ``CONFIG_NR_LRU_GENS`` to a larger value to support
+ more generations for ``Working set estimation`` and
+ ``Proactive reclaim``.
+
+:Optional: Change ``CONFIG_TIERS_PER_GEN`` to a larger value to
+ support more tiers, which generally provide better protection for
+ page cache when under heavy buffered I/O workloads.
+
+:Optional: Set ``CONFIG_LRU_GEN_STATS=y`` to enable full stats for
+ debugging. See ``Debugfs interface``.
+
 :Debugfs interface: ``/sys/kernel/debug/lru_gen`` has the following
  format:
  ::
diff --git a/mm/Kconfig b/mm/Kconfig
index 356f4f2c779e..8a33605917f5 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -900,6 +900,54 @@  config IO_MAPPING
 config SECRETMEM
 	def_bool ARCH_HAS_SET_DIRECT_MAP && !EMBEDDED
 
+# multigenerational lru {
+config LRU_GEN
+	bool "Multigenerational LRU"
+	depends on MMU
+	# the following options can use up the spare bits in page flags
+	depends on !MAXSMP && (64BIT || !SPARSEMEM || SPARSEMEM_VMEMMAP)
+	help
+	  A high performance LRU implementation for memory overcommit. See
+	  Documentation/vm/multigen_lru.rst for details.
+
+config LRU_GEN_ENABLED
+	bool "Enable by default"
+	depends on LRU_GEN
+	help
+	  This option enables the multigenerational lru by default.
+
+config NR_LRU_GENS
+	int "Max number of generations"
+	depends on LRU_GEN
+	range 4 31
+	default 4
+	help
+	  This option uses order_base_2(N+1) bits in page flags.
+
+	  Do not configure more generations than you plan to use. They have a
+	  per-memcg and per-node memory overhead.
+
+config TIERS_PER_GEN
+	int "Number of tiers per generation"
+	depends on LRU_GEN
+	range 2 5
+	default 4
+	help
+	  This option uses N-2 bits in page flags.
+
+	  Larger values generally provide better protection for page cache when
+	  under heavy buffered I/O workloads.
+
+config LRU_GEN_STATS
+	bool "Full stats for debugging"
+	depends on LRU_GEN
+	help
+	  This option keeps historical stats for evicted generations.
+
+	  Do not enable full stats unless you plan to look at them. They have a
+	  per-memcg and per-node memory overhead.
+# }
+
 source "mm/damon/Kconfig"
 
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