Re: [PATCH v9 13/14] mm: multi-gen LRU: admin guide

[Mike Rapoport]

Hi,

On Tue, Mar 08, 2022 at 07:12:30PM -0700, Yu Zhao wrote:
> Add an admin guide.
> 
> Signed-off-by: Yu Zhao <yuzhao@xxxxxxxxxx>
> Acked-by: Brian Geffon <bgeffon@xxxxxxxxxx>
> Acked-by: Jan Alexander Steffens (heftig) <heftig@xxxxxxxxxxxxx>
> Acked-by: Oleksandr Natalenko <oleksandr@xxxxxxxxxxxxxx>
> Acked-by: Steven Barrett <steven@xxxxxxxxxxxx>
> Acked-by: Suleiman Souhlal <suleiman@xxxxxxxxxx>
> Tested-by: Daniel Byrne <djbyrne@xxxxxxx>
> Tested-by: Donald Carr <d@xxxxxxxxxxxxxxx>
> Tested-by: Holger Hoffstätte <holger@xxxxxxxxxxxxxxxxxxxxxx>
> Tested-by: Konstantin Kharlamov <Hi-Angel@xxxxxxxxx>
> Tested-by: Shuang Zhai <szhai2@xxxxxxxxxxxxxxxx>
> Tested-by: Sofia Trinh <sofia.trinh@edi.works>
> Tested-by: Vaibhav Jain <vaibhav@xxxxxxxxxxxxx>
> ---
>  Documentation/admin-guide/mm/index.rst        |   1 +
>  Documentation/admin-guide/mm/multigen_lru.rst | 146 ++++++++++++++++++
>  mm/Kconfig                                    |   3 +-
>  3 files changed, 149 insertions(+), 1 deletion(-)
>  create mode 100644 Documentation/admin-guide/mm/multigen_lru.rst
> 
> diff --git a/Documentation/admin-guide/mm/index.rst b/Documentation/admin-guide/mm/index.rst
> index c21b5823f126..2cf5bae62036 100644
> --- a/Documentation/admin-guide/mm/index.rst
> +++ b/Documentation/admin-guide/mm/index.rst
> @@ -32,6 +32,7 @@ the Linux memory management.
>     idle_page_tracking
>     ksm
>     memory-hotplug
> +   multigen_lru
>     nommu-mmap
>     numa_memory_policy
>     numaperf
> diff --git a/Documentation/admin-guide/mm/multigen_lru.rst b/Documentation/admin-guide/mm/multigen_lru.rst
> new file mode 100644
> index 000000000000..4ea6a801dc56
> --- /dev/null
> +++ b/Documentation/admin-guide/mm/multigen_lru.rst
> @@ -0,0 +1,146 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +=============
> +Multi-Gen LRU
> +=============

I'm still missing an opening paragraph the explains what is Multi-gen LRU
and why users would want it.

Something like 

  Multi-gen LRU is an efficient mechanism for page reclamation.

More details are of course welcome :)


> +Quick start
> +===========
> +Build the kernel with the following configurations.
> +
> +* ``CONFIG_LRU_GEN=y``
> +* ``CONFIG_LRU_GEN_ENABLED=y``
> +
> +All set!
> +
> +Runtime options
> +===============
> +``/sys/kernel/mm/lru_gen/`` contains stable ABIs described in the
> +following subsections.
> +
> +Kill switch
> +-----------
> +``enable`` accepts different values to enable or disabled the

                                                   ^ disable

> +following components. The default value of this file depends on
> +``CONFIG_LRU_GEN_ENABLED``. All the components should be enabled
> +unless some of them have unforeseen side effects. Writing to
> +``enable`` has no effect when a component is not supported by the
> +hardware, and valid values will be accepted even when the main switch
> +is off.
> +
> +====== ===============================================================
> +Values Components
> +====== ===============================================================
> +0x0001 The main switch for the multi-gen LRU.
> +0x0002 Clearing the accessed bit in leaf page table entries in large
> +       batches, when MMU sets it (e.g., on x86). This behavior can
> +       theoretically worsen lock contention (mmap_lock). If it is
> +       disabled, the multi-gen LRU will suffer a minor performance
> +       degradation.
> +0x0004 Clearing the accessed bit in non-leaf page table entries as
> +       well, when MMU sets it (e.g., on x86). This behavior was not
> +       verified on x86 varieties other than Intel and AMD. If it is
> +       disabled, the multi-gen LRU will suffer a negligible
> +       performance degradation.
> +[yYnN] Apply to all the components above.
> +====== ===============================================================
> +
> +E.g.,
> +::
> +
> +    echo y >/sys/kernel/mm/lru_gen/enabled
> +    cat /sys/kernel/mm/lru_gen/enabled
> +    0x0007
> +    echo 5 >/sys/kernel/mm/lru_gen/enabled
> +    cat /sys/kernel/mm/lru_gen/enabled
> +    0x0005
> +
> +Thrashing prevention
> +--------------------
> +Personal computers are more sensitive to thrashing because it can
> +cause janks (lags when rendering UI) and negatively impact user
> +experience. The multi-gen LRU offers thrashing prevention to the
> +majority of laptop and desktop users who do not have ``oomd``.
> +
> +Users can write ``N`` to ``min_ttl_ms`` to prevent the working set of
> +``N`` milliseconds from getting evicted. The OOM killer is triggered
> +if this working set cannot be kept in memory. In other words, this
> +option works as an adjustable pressure relief valve, and when open, it
> +terminates applications that are hopefully not being used.
> +
> +Based on the average human detectable lag (~100ms), ``N=1000`` usually
> +eliminates intolerable janks due to thrashing. Larger values like
> +``N=3000`` make janks less noticeable at the risk of premature OOM
> +kills.

What is the default value of min_ttl_ms?

> +
> +Experimental features
> +=====================

-- 
Sincerely yours,
Mike.