linux-kernel - [PATCH v2 4/4] mm/ksm: document ksm advisor and its sysfs knobs

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Message-Id: <20231028000945.2428830-5-shr@devkernel.io>
Date:   Fri, 27 Oct 2023 17:09:45 -0700
From:   Stefan Roesch <shr@...kernel.io>
To:     kernel-team@...com
Cc:     shr@...kernel.io, akpm@...ux-foundation.org, david@...hat.com,
        hannes@...xchg.org, riel@...riel.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: [PATCH v2 4/4] mm/ksm: document ksm advisor and its sysfs knobs

This documents the KSM advisor and its new knobs in /sys/fs/kernel/mm.

Signed-off-by: Stefan Roesch <shr@...kernel.io>
---
 Documentation/admin-guide/mm/ksm.rst | 66 ++++++++++++++++++++++++++++
 1 file changed, 66 insertions(+)

diff --git a/Documentation/admin-guide/mm/ksm.rst b/Documentation/admin-guide/mm/ksm.rst
index e59231ac6bb7..7e956692b656 100644
--- a/Documentation/admin-guide/mm/ksm.rst
+++ b/Documentation/admin-guide/mm/ksm.rst
@@ -164,6 +164,33 @@ smart_scan
         optimization is enabled.  The ``pages_skipped`` metric shows how
         effective the setting is.
 
+advisor_mode
+        The ``advisor_mode`` selects the current advisor. Two modes are
+        supported: 0 (None) and 1 (Scan time). The default is None. By
+        setting ``advisor_mode`` to 1, the scan time advisor is enabled.
+        The section about ``advisor`` explains in detail how the scan time
+        advisor works.
+
+advisor_min_cpu
+        specifies the lower limit of the cpu percent usage of the ksmd
+        background thread. The default is 15.
+
+adivsor_max_cpu
+        specifies the upper limit of the cpu percent usage of the ksmd
+        background thread. The default is 70.
+
+advisor_target_scan_time
+        specifies the target scan time in seconds to scan all the candidate
+        pages. The default value is 200 seconds.
+
+advisor_min_pages
+        specifies the lower limit of the ``pages_to_scan`` parameter of the
+        scan time advisor. The default is 500.
+
+adivsor_max_pages
+        specifies the upper limit of the ``pages_to_scan`` parameter of the
+        scan time advisor. The default is 30000.
+
 The effectiveness of KSM and MADV_MERGEABLE is shown in ``/sys/kernel/mm/ksm/``:
 
 general_profit
@@ -263,6 +290,45 @@ ksm_swpin_copy
 	note that KSM page might be copied when swapping in because do_swap_page()
 	cannot do all the locking needed to reconstitute a cross-anon_vma KSM page.
 
+Advisor
+=======
+
+The number of candidate pages for KSM is dynamic. It can be often observed
+that during the startup of an application more candidate pages need to be
+processed. Without an advisor the ``pages_to_scan`` parameter needs to be
+sized for the maximum number of candidate pages. The scan time advisor can
+changes the ``pages_to_scan`` parameter based on demand.
+
+The advisor can be enabled, so KSM can automatically adapt to changes in the
+number of candidate pages to scan. Two advisors are implemented: 0 (None) and
+1 (Scan time). With None no advisor is enabled. The default is None.
+
+The Scan time advisor changes the ``pages_to_scan`` parameter based on the
+observed scan times. The possible values for the ``pages_to_scan`` parameter is
+limited by the ``advisor_min_pages`` and ``advisor_max_pages`` parameters. In
+addition there is also the ``advisor_target_scan_time`` parameter. This
+parameter sets the target time to scan all the KSM candidate pages. The
+parameter ``advisor_target_scan_time`` decides how aggressive the scan time
+advisor scans candidate pages. Lower values make the scan time advisor to scan
+more aggresively. This is the most important parameter for the configuration of
+the scan time advisor.
+
+The Scan time advisor changes the ``pages_to_scan`` parameter based on the
+observed scan times. The possible values for the ``pages_to_scan`` parameter is
+limited by the ``advisor_min_cpu`` and ``advisor_max_cpu`` parameters. In
+addition there is also the ``advisor_target_scan_time`` parameter. This
+parameter sets the target time to scan all the KSM candidate pages. The
+parameter ``advisor_target_scan_time`` decides how aggressive the scan time
+advisor scans candidate pages. Lower values make the scan time advisor to scan
+more aggresively. This is the most important parameter for the configuration of
+the scan time advisor.
+
+The initial value and the maximum value can be changed with ``advisor_min_pages``
+and ``advisor_max_pages``. The default values are sufficient for most workloads.
+
+The ``pages_to_scan`` parameter is re-calculated after a scan has been completed.
+
+
 --
 Izik Eidus,
 Hugh Dickins, 17 Nov 2009
-- 
2.39.3