linux-kernel - [PATCH 0/2] [tip: sched/core] sched: Disable PLACE_LAG and RUN_TO

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Message-ID: <20241017052000.99200-1-cpru@amazon.com>
Date: Thu, 17 Oct 2024 00:19:58 -0500
From: Cristian Prundeanu <cpru@...zon.com>
To: <linux-tip-commits@...r.kernel.org>
CC: <linux-kernel@...r.kernel.org>, Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, <x86@...nel.org>,
	<linux-arm-kernel@...ts.infradead.org>, Bjoern Doebel <doebel@...zon.com>,
	Hazem Mohamed Abuelfotoh <abuehaze@...zon.com>, Geoff Blake
	<blakgeof@...zon.com>, Ali Saidi <alisaidi@...zon.com>, Csaba Csoma
	<csabac@...zon.com>, Cristian Prundeanu <cpru@...zon.com>
Subject: [PATCH 0/2] [tip: sched/core] sched: Disable PLACE_LAG and RUN_TO_PARITY and move them to sysctl

This patchset disables the scheduler features PLACE_LAG and RUN_TO_PARITY 
and moves them to sysctl.

Replacing CFS with the EEVDF scheduler in kernel 6.6 introduced 
significant performance degradation in multiple database-oriented 
workloads. This degradation manifests in all kernel versions using EEVDF, 
across multiple Linux distributions, hardware architectures (x86_64, 
aarm64, amd64), and CPU generations.

For example, running mysql+hammerdb results in a 12-17% throughput 
reduction and 12-18% latency increase compared to kernel 6.5 (using 
default scheduler settings everywhere). The magnitude of this performance 
impact is comparable to the average performance difference of a CPU 
generation over its predecessor.

Testing combinations of available scheduler features showed that the 
largest improvement (short of disabling all EEVDF features) came from 
disabling both PLACE_LAG and RUN_TO_PARITY:

Kernel   | default  | NO_PLACE_LAG and
aarm64   | config   | NO_RUN_TO_PARITY
---------+----------+-----------------
6.5      | baseline |  N/A
6.6      | -13.2%   | -6.8%
6.7      | -13.1%   | -6.0%
6.8      | -12.3%   | -6.5%
6.9      | -12.7%   | -6.9%
6.10     | -13.5%   | -5.8%
6.11     | -12.6%   | -5.8%
6.12-rc2 | -12.2%   | -8.9%
---------+----------+-----------------

Kernel   | default  | NO_PLACE_LAG and
x86_64   | config   | NO_RUN_TO_PARITY
---------+----------+-----------------
6.5      | baseline |  N/A
6.6      | -16.8%   | -10.8%
6.7      | -16.4%   |  -9.9%
6.8      | -17.2%   |  -9.5%
6.9      | -17.4%   |  -9.7%
6.10     | -16.5%   |  -9.0%
6.11     | -15.0%   |  -8.5%
6.12-rc2 | -12.7%   | -10.9%
---------+----------+-----------------

While the long term approach is debugging and fixing the scheduler 
behavior, algorithm changes to address performance issues of this nature 
are specialized (and likely prolonged or open-ended) research. Until a 
change is identified which fixes the performance degradation, in the 
interest of a better out-of-the-box performance: (1) disable these 
features by default, and (2) expose these values in sysctl instead of 
debugfs, so they can be more easily persisted across reboots.

Cristian Prundeanu (2):
  sched: Disable PLACE_LAG and RUN_TO_PARITY
  sched: Move PLACE_LAG and RUN_TO_PARITY to sysctl

 include/linux/sched/sysctl.h |  8 ++++++++
 kernel/sched/core.c          | 13 +++++++++++++
 kernel/sched/fair.c          |  5 +++--
 kernel/sched/features.h      | 10 ----------
 kernel/sysctl.c              | 20 ++++++++++++++++++++
 5 files changed, 44 insertions(+), 12 deletions(-)

-- 
2.40.1