[PATCH 0/1] sched: Restore PREEMPT_NONE as default

Salvatore Dipietro
2026-04-03T19:20:48Z

We are reporting a throughput and latency regression on PostgreSQL
pgbench (simple-update) on arm64 caused by commit 7dadeaa6e851
("sched: Further restrict the preemption modes") introduced in
v7.0-rc1.

The regression manifests as a 0.51x throughput drop on a pgbench
simple-update workload with 1024 clients on a 96-vCPU
(AWS EC2 m8g.24xlarge) Graviton4 arm64 system. Perf profiling
shows 55% of CPU time is consumed spinning in PostgreSQL's
userspace spinlock (s_lock()) under PREEMPT_LAZY:

|- 56.03% - StartReadBuffer
|- 55.93% - GetVictimBuffer
|- 55.93% - StrategyGetBuffer
|- 55.60% - s_lock <<<< 55% of time
| |- 0.39% - el0t_64_irq
| |- 0.10% - perform_spin_delay
|- 0.08% - LockBufHdr
|- 0.07% - hash_search_with_hash_value
|- 0.40% - WaitReadBuffers

1. Test environment
___________________

Hardware: 1x AWS EC2 m8g.24xlarge (12x 1TB IO2 32000 iops RAID0 XFS)
OS: AL2023 (ami-03a8d3251f401ffca)
Kernel: next-20260331
Database: PostgreSQL 17
Workload: pgbench simple-update
1024 clients, 96 threads, 1200s duration
scale factor 8470, fillfactor=90, prepared protocol

2. Results
__________

Configuration Run 1 Run 2 Run 3 Average vs Base
__________________ _________ _________ _________ _________ _______
BASELINE 47242.39 53369.18 51644.29 50751.96 1.00x
w/ revert 92906.62 103976.03 98814.94 98565.86 1.94x

3. Reproduction
_______________

On the AWS EC2 m8g.24xlarge, install and run the PostgreSQL
database using the repro-collection repository like:

# Reproducer code:
git clone https://github.com/aws/repro-collection.git ~/repro-collection

# Setup and start PostgreSQL server in terminal 1:
~/repro-collection/run.sh postgresql SUT --ldg=127.0.0.1

# Run pgbench load generator in terminal 2:
PGBENCH_SCALE=8470 \
PGBENCH_INIT_EXTRA_ARGS="--fillfactor=90" \
PGBENCH_CLIENTS=1024 \
PGBENCH_THREADS=96 \
PGBENCH_DURATION=1200 \
PGBENCH_BUILTIN=simple-update \
PGBENCH_PROTOCOL=prepared \
~/repro-collection/run.sh postgresql LDG --sut=127.0.0.1

Salvatore Dipietro (1):
sched: Restore PREEMPT_NONE as default

kernel/Kconfig.preempt | 3 ---
1 file changed, 3 deletions(-)

base-commit: 9147566d801602c9e7fc7f85e989735735bf38ba
--
2.50.1 (Apple Git-155)

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