linux-kernel - [PATCH v5 4/4] mm/ksm: document ksm advisor and its sysfs knobs

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Message-Id: <20231218231054.1625219-5-shr@devkernel.io>
Date: Mon, 18 Dec 2023 15:10:54 -0800
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 v5 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>
Acked-by: David Hildenbrand <david@...hat.com>
---
 Documentation/admin-guide/mm/ksm.rst | 55 ++++++++++++++++++++++++++++
 1 file changed, 55 insertions(+)

diff --git a/Documentation/admin-guide/mm/ksm.rst b/Documentation/admin-guide/mm/ksm.rst
index e59231ac6bb71..a639cac124777 100644
--- a/Documentation/admin-guide/mm/ksm.rst
+++ b/Documentation/admin-guide/mm/ksm.rst
@@ -80,6 +80,9 @@ pages_to_scan
         how many pages to scan before ksmd goes to sleep
         e.g. ``echo 100 > /sys/kernel/mm/ksm/pages_to_scan``.
 
+        The pages_to_scan value cannot be changed if ``advisor_mode`` has
+        been set to scan-time.
+
         Default: 100 (chosen for demonstration purposes)
 
 sleep_millisecs
@@ -164,6 +167,29 @@ 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: none and scan-time. The default is none. By setting
+        ``advisor_mode`` to scan-time, the scan time advisor is enabled.
+        The section about ``advisor`` explains in detail how the scan time
+        advisor works.
+
+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_to_scan
+        specifies the lower limit of the ``pages_to_scan`` parameter of the
+        scan time advisor. The default is 500.
+
+adivsor_max_pages_to_scan
+        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 +289,35 @@ 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: none and
+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_max_cpu`` parameter. 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_to_scan`` and ``advisor_max_pages_to_scan``. The default
+values are sufficient for most workloads and use cases.
+
+The ``pages_to_scan`` parameter is re-calculated after a scan has been completed.
+
+
 --
 Izik Eidus,
 Hugh Dickins, 17 Nov 2009
-- 
2.39.3