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

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Message-ID: <YinvPMIP4oEwCo6T@kernel.org>
Date:   Thu, 10 Mar 2022 14:29:48 +0200
From:   Mike Rapoport <rppt@...nel.org>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Aneesh Kumar <aneesh.kumar@...ux.ibm.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Hillf Danton <hdanton@...a.com>, Jens Axboe <axboe@...nel.dk>,
        Jesse Barnes <jsbarnes@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Michael Larabel <Michael@...haellarabel.com>,
        Michal Hocko <mhocko@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Will Deacon <will@...nel.org>,
        Ying Huang <ying.huang@...el.com>,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        page-reclaim@...gle.com, x86@...nel.org,
        Brian Geffon <bgeffon@...gle.com>,
        Jan Alexander Steffens <heftig@...hlinux.org>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Steven Barrett <steven@...uorix.net>,
        Suleiman Souhlal <suleiman@...gle.com>,
        Daniel Byrne <djbyrne@....edu>,
        Donald Carr <d@...os-reins.com>,
        Holger Hoffstätte 
        <holger@...lied-asynchrony.com>,
        Konstantin Kharlamov <Hi-Angel@...dex.ru>,
        Shuang Zhai <szhai2@...rochester.edu>,
        Sofia Trinh <sofia.trinh@....works>,
        Vaibhav Jain <vaibhav@...ux.ibm.com>
Subject: Re: [PATCH v9 13/14] mm: multi-gen LRU: admin guide

Hi,

On Tue, Mar 08, 2022 at 07:12:30PM -0700, Yu Zhao wrote:
> Add an admin guide.
> 
> Signed-off-by: Yu Zhao <yuzhao@...gle.com>
> Acked-by: Brian Geffon <bgeffon@...gle.com>
> Acked-by: Jan Alexander Steffens (heftig) <heftig@...hlinux.org>
> Acked-by: Oleksandr Natalenko <oleksandr@...alenko.name>
> Acked-by: Steven Barrett <steven@...uorix.net>
> Acked-by: Suleiman Souhlal <suleiman@...gle.com>
> Tested-by: Daniel Byrne <djbyrne@....edu>
> Tested-by: Donald Carr <d@...os-reins.com>
> Tested-by: Holger Hoffstätte <holger@...lied-asynchrony.com>
> Tested-by: Konstantin Kharlamov <Hi-Angel@...dex.ru>
> Tested-by: Shuang Zhai <szhai2@...rochester.edu>
> Tested-by: Sofia Trinh <sofia.trinh@....works>
> Tested-by: Vaibhav Jain <vaibhav@...ux.ibm.com>
> ---
>  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.