Re: [PATCH 0/2] [tip: sched/core] sched: Disable PLACE_LAG and RUN_TO_PARITY and move them to sysctl

From: Joseph Salisbury
Date: Thu Nov 14 2024 - 15:11:29 EST





On 10/17/24 01:19, Cristian Prundeanu wrote:
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(-)

Hi Cristian,

This is a confirmation that we are also seeing a 9% performance regression with the TPCC benchmark after v6.6-rc1.  We narrowed down the regression was caused due to commit:
86bfbb7ce4f6 ("sched/fair: Add lag based placement")

This regression was reported via this thread:
https://lore.kernel.org/lkml/1c447727-92ed-416c-bca1-a7ca0974f0df@xxxxxxxxxx/

Phil Auld suggested to try turning off the PLACE_LAG sched feature. We tested with NO_PLACE_LAG and can confirm it brought back 5% of the performance loss.  We do not yet know what effect NO_PLACE_LAG will have on other benchmarks, but it indeed helps TPCC.

Thanks for the work to move PLACE_LAG and RUN_TO_PARITY to sysctl!

Joe