Date:   Fri, 25 Feb 2022 15:32:47 +0800
From:   xhao@...ux.alibaba.com
To:     SeongJae Park <sj@...nel.org>, akpm@...ux-foundation.org
Cc:     corbet@....net, skhan@...uxfoundation.org, rientjes@...gle.com,
        linux-damon@...zon.com, linux-mm@...ck.org,
        linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/12] Introduce DAMON sysfs interface

Hi SeongJae:

On 2/23/22 11:20 PM, SeongJae Park wrote:
> Chages from Previous Version (RFC)
> ==================================
>
> Compared to the RFC version of this patchset
> (https://lore.kernel.org/linux-mm/20220217161938.8874-1-sj@kernel.org/), this
> version contains below changes.
>
> - Implement all DAMON debugfs interface providing features
> - Writeup documents
> - Add more selftests
>
> Introduction
> ============
>
> DAMON's debugfs-based user interface (DAMON_DBGFS) served very well, so far.
> However, it unnecessarily depends on debugfs, while DAMON is not aimed to be
> used for only debugging.  Also, the interface receives multiple values via one
> file.  For example, schemes file receives 18 values.  As a result, it is
> inefficient, hard to be used, and difficult to be extended.  Especially,
> keeping backward compatibility of user space tools is getting only challenging.
> It would be better to implement another reliable and flexible interface and
> deprecate DAMON_DBGFS in long term.
>
> For the reason, this patchset introduces a sysfs-based new user interface of
> DAMON.  The idea of the new interface is, using directory hierarchies and
> having one dedicated file for each value.  For a short example, users can do
> the virtual address monitoring via the interface as below:
>
>      # cd /sys/kernel/mm/damon/admin/
>      # echo 1 > kdamonds/nr
>      # echo 1 > kdamonds/0/contexts/nr
>      # echo vaddr > kdamonds/0/contexts/0/operations
>      # echo 1 > kdamonds/0/contexts/0/targets/nr
>      # echo $(pidof <workload>) > kdamonds/0/contexts/0/targets/0/pid
>      # echo on > kdamonds/0/state
>
> A brief representation of the files hierarchy of DAMON sysfs interface is as
> below.  Childs are represented with indentation, directories are having '/'
> suffix, and files in each directory are separated by comma.
>
>      /sys/kernel/mm/damon/admin
>      │ kdamonds/nr
>      │ │ 0/state,pid
>      │ │ │ contexts/nr
>      │ │ │ │ 0/operations
>      │ │ │ │ │ monitoring_attrs/
>      │ │ │ │ │ │ intervals/sample_us,aggr_us,update_us
>      │ │ │ │ │ │ nr_regions/min,max
>      │ │ │ │ │ targets/nr
>      │ │ │ │ │ │ 0/pid
>      │ │ │ │ │ │ │ regions/nr
>      │ │ │ │ │ │ │ │ 0/start,end
>      │ │ │ │ │ │ │ │ ...
>      │ │ │ │ │ │ ...
>      │ │ │ │ │ schemes/nr
>      │ │ │ │ │ 0/action
>      │ │ │ │ │ │ access_pattern/
>      │ │ │ │ │ │ │ sz/min,max
>      │ │ │ │ │ │ │ nr_accesses/min,max
>      │ │ │ │ │ │ │ age/min,max
>      │ │ │ │ │ │ quotas/ms,sz,reset_interval_ms
>      │ │ │ │ │ │ │ weights/sz,nr_accesses,age
>      │ │ │ │ │ │ watermarks/metric,interval_us,high,mid,low
>      │ │ │ │ │ │ stats/nr_tried,sz_tried,nr_applied,sz_applied,qt_exceeds
>      │ │ │ │ │ ...
>      │ │ ...
>
> Detailed usage of the files will be described in the final Documentation patch
> of this patchset.

The introduction of the sys DAMON interface makes DAMON seem more 
hierarchical, but it brings a problem. From a user's perspective,

I find it difficult to operate. For example:

step one:

echo xxx > /sys/kernel/mm/damon/admin/kdamonds/0/contexts/0/targets/nr

step two:

echo /sys/kernel/mm/damon/admin/kdamonds/0/contexts/0/targets/nr/1/pid

echo /sys/kernel/mm/damon/admin/kdamonds/0/contexts/0/targets/nr/0/pid

.........

Alas, it is really too troublesome to operate, can you make it as simple 
as possible, perhaps by referring to the implementation of cgroup.

> Main Difference Between DAMON_DBGFS and DAMON_SYSFS
> ---------------------------------------------------
>
> At the moment, DAMON_DBGFS and DAMON_SYSFS provides same features.  One
> important difference between them is their exclusiveness.  DAMON_DBGFS works in
> an exclusive manner, so that no DAMON worker thread (kdamond) in the system can
> run concurrently and interfere somehow.  For the reason, DAMON_DBGFS asks users
> to construct all monitoring contexts and start them at once.  It's not a big
> problem but makes the operation a little bit complex and unflexible.
>
> For more flexible usage, DAMON_SYSFS moves the responsibility of preventing any
> possible interference to the admins and work in a non-exclusive manner.  That
> is, users can configure and start contexts one by one.  Note that DAMON
> respects both exclusive groups and non-exclusive groups of contexts, in a
> manner similar to that of reader-writer locks.  That is, if any exclusive
> monitoring contexts (e.g., contexts that started via DAMON_DBGFS) are running,
> DAMON_SYSFS does not start new contexts, and vice versa.
>
> Future Plan of DAMON_DBGFS Deprecation
> ======================================
>
> Once this patchset is merged, DAMON_DBGFS development will be frozen.  That is,
> we will maintain it to work as is now so that no users will be break.  But, it
> will not be extended to provide any new feature of DAMON.  The support will be
> continued only until next LTS release.  After that, we will drop DAMON_DBGFS.
>
> User-space Tooling Compatibility
> --------------------------------
>
> As DAMON_SYSFS provides all features of DAMON_DBGFS, all user space tooling can
> move to DAMON_SYSFS.  As we will continue supporting DAMON_DBGFS until next LTS
> kernel release, user space tools would have enough time to move to DAMON_SYSFS.
>
> The official user space tool, damo[1], is already supporting both DAMON_SYSFS
> and DAMON_DBGFS.  Both correctness tests[2] and performance tests[3] of DAMON
> using DAMON_SYSFS also passed.
>
> [1] https://github.com/awslabs/damo
> [2] https://github.com/awslabs/damon-tests/tree/master/corr
> [3] https://github.com/awslabs/damon-tests/tree/master/perf
>
> Complete Git Tree
> =================
>
> You can get the complete git tree from
> https://git.kernel.org/sj/h/damon/sysfs/patches/v1.
>
> Sequence of Patches
> ===================
>
> First two patches (patches 1-2) make core changes for DAMON_SYSFS.  The first
> one (patch 1) allows non-exclusive DAMON contexts so that DAMON_SYSFS can work
> in non-exclusive mode, while the second one (patch 2) adds size of DAMON enum
> types so that DAMON API users can safely iterate the enums.
>
> Third patch (patch 3) implements basic sysfs stub for virtual address spaces
> monitoring.  Note that this implements only sysfs files and DAMON is not
> linked.  Fourth patch (patch 4) links the DAMON_SYSFS to DAMON so that users
> can control DAMON using the sysfs files.
>
> Following six patches (patches 5-10) implements other DAMON features that
> DAMON_DBGFS supports one by one (physical address space monitoring, DAMON-based
> operation schemes, schemes quotas, schemes prioritization weights, schemes
> watermarks, and schemes stats).
>
> Following patch (patch 11) adds a simple selftest for DAMON_SYSFS, and the
> final one (patch 12) documents DAMON_SYSFS.
>
> SeongJae Park (12):
>    mm/damon/core: Allow non-exclusive DAMON start/stop
>    mm/damon/core: Add number of each enum type values
>    mm/damon: Implement a minimal stub for sysfs-based DAMON interface
>    mm/damon/sysfs: Link DAMON for virtual address spaces monitoring
>    mm/damon/sysfs: Support physical address space monitoring
>    mm/damon/sysfs: Support DAMON-based Operation Schemes
>    mm/damon/sysfs: Support DAMOS quotas
>    mm/damon/sysfs: Support schemes prioritization weights
>    mm/damon/sysfs: Support DAMOS watermarks
>    mm/damon/sysfs: Support DAMOS stats
>    selftests/damon: Add a test for DAMON sysfs interface
>    Docs/admin-guide/mm/damon/usage: Document DAMON sysfs interface
>
>   Documentation/admin-guide/mm/damon/usage.rst |  349 ++-
>   include/linux/damon.h                        |    6 +-
>   mm/damon/Kconfig                             |    7 +
>   mm/damon/Makefile                            |    1 +
>   mm/damon/core.c                              |   23 +-
>   mm/damon/dbgfs.c                             |    2 +-
>   mm/damon/reclaim.c                           |    2 +-
>   mm/damon/sysfs.c                             | 2684 ++++++++++++++++++
>   tools/testing/selftests/damon/Makefile       |    1 +
>   tools/testing/selftests/damon/sysfs.sh       |  306 ++
>   10 files changed, 3364 insertions(+), 17 deletions(-)
>   create mode 100644 mm/damon/sysfs.c
>   create mode 100755 tools/testing/selftests/damon/sysfs.sh
>
-- 
Best Regards!
Xin Hao

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