Re: [PATCH v2 8/8] Docs/admin-guide/mm/workingset_report: document sysfs and memcg interfaces

[Randy Dunlap]

Hi,

Just a couple of nits below:

On 6/3/24 7:05 PM, Yuanchu Xie wrote:
> Add workingset reporting documentation for better discoverability of
> its sysfs and memcg interfaces. Also document the required kernel
> config to enable workingset reporting.
> 
> Signed-off-by: Yuanchu Xie <yuanchu@xxxxxxxxxx>
> ---
>  Documentation/admin-guide/mm/index.rst        |   1 +
>  .../admin-guide/mm/workingset_report.rst      | 105 ++++++++++++++++++
>  2 files changed, 106 insertions(+)
>  create mode 100644 Documentation/admin-guide/mm/workingset_report.rst
> 
> diff --git a/Documentation/admin-guide/mm/index.rst b/Documentation/admin-guide/mm/index.rst
> index 1f883abf3f00..fba987de8997 100644
> --- a/Documentation/admin-guide/mm/index.rst
> +++ b/Documentation/admin-guide/mm/index.rst
> @@ -41,4 +41,5 @@ the Linux memory management.
>     swap_numa
>     transhuge
>     userfaultfd
> +   workingset_report
>     zswap
> diff --git a/Documentation/admin-guide/mm/workingset_report.rst b/Documentation/admin-guide/mm/workingset_report.rst
> new file mode 100644
> index 000000000000..f455ae93b30e
> --- /dev/null
> +++ b/Documentation/admin-guide/mm/workingset_report.rst
> @@ -0,0 +1,105 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +=================
> +Workingset Report
> +=================
> +Workingset report provides a view of memory coldness in user-defined
> +time intervals, i.e. X bytes are Y milliseconds cold. It breaks down
> +the user pages in the system per-NUMA node, per-memcg, for both
> +anonymous and file pages into histograms that look like:
> +::
> +
> +    1000 anon=137368 file=24530
> +    20000 anon=34342 file=0
> +    30000 anon=353232 file=333608
> +    40000 anon=407198 file=206052
> +    9223372036854775807 anon=4925624 file=892892
> +
> +The workingset reports can be used to drive proactive reclaim, by
> +identifying the number of cold bytes in a memcg, then writing to
> +``memory.reclaim``.
> +
> +Quick start
> +===========
> +Build the kernel with the following configurations. The report relies
> +on Multi-gen LRU for page coldness.
> +
> +* ``CONFIG_LRU_GEN=y``
> +* ``CONFIG_LRU_GEN_ENABLED=y``
> +* ``CONFIG_WORKINGSET_REPORT=y``
> +
> +Optionally, the aging kernel daemon can be enabled with the following
> +configuration.
> +* ``CONFIG_LRU_GEN_ENABLED=y``
> +
> +Sysfs interfaces
> +================
> +``/sys/devices/system/node/nodeX/page_age`` provides a per-node page
> +age histogram, showing an aggregate of the node's lruvecs.
> +Reading this file causes a hierarchical aging of all lruvecs, scanning
> +pages and creates a new Multi-gen LRU generation in each lruvec.
> +For example:
> +::
> +
> +    1000 anon=0 file=0
> +    2000 anon=0 file=0
> +    100000 anon=5533696 file=5566464
> +    18446744073709551615 anon=0 file=0
> +
> +``/sys/devices/system/node/nodeX/page_age_interval`` is a comma
> +separated list of time in milliseconds that configures what the page
> +age histogram uses for aggregation. For the above histogram,
> +the intervals are:
> +::
> +    1000,2000,100000
> +
> +``/sys/devices/system/node/nodeX/workingset_report/refresh_interval``
> +defines the amount of time the report is valid for in milliseconds.
> +When a report is still valid, reading the ``page_age`` file shows
> +the existing valid report, instead of generating a new one.
> +
> +``/sys/devices/system/node/nodeX/workingset_report/report_threshold``
> +specifies how often the userspace agent can be notified for node
> +memory pressure, in milliseconds. When a node reaches its low
> +watermarks and wakes up kswapd, programs waiting on ``page_age`` are
> +woken up so they can read the histogram and make policy decisions.
> +
> +Memcg interface
> +===============
> +While ``page_age_interval`` is defined per-node in sysfs. ``page_age``,

                                                      sysfs,

> +``refresh_interval`` and ``report_threshold`` are available per-memcg.
> +
> +``/sys/fs/cgroup/.../memory.workingset.page_age``
> +The memcg equivalent of the sysfs workingset page age histogram,

                                                       no comma   ^

> +breaks down the workingset of this memcg and its children into
> +page age intervals. Each node is prefixed with a node header and
> +a newline. Non-proactive direct reclaim on this memcg can also
> +wake up userspace agents that are waiting on this file.
> +e.g.
> +::
> +
> +    N0
> +    1000 anon=0 file=0
> +    2000 anon=0 file=0
> +    3000 anon=0 file=0
> +    4000 anon=0 file=0
> +    5000 anon=0 file=0
> +    18446744073709551615 anon=0 file=0
> +
> +``/sys/fs/cgroup/.../memory.workingset.refresh_interval``
> +The memcg equivalent of the sysfs refresh interval. A per-node
> +number of how much time a page age histogram is valid for, in
> +milliseconds.
> +e.g.
> +::
> +
> +    echo N0=2000 > memory.workingset.refresh_interval
> +
> +``/sys/fs/cgroup/.../memory.workingset.report_threshold``
> +The memcg equivalent of the sysfs report threshold. A per-node
> +number of how often userspace agent waiting on the page age
> +histogram can be woken up, in milliseconds.
> +e.g.
> +::
> +
> +    echo N0=1000 > memory.workingset.report_threshold

-- 
#Randy
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