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

[Salvatore Dipietro <dipiets@amazon.it>]

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)




AMAZON DEVELOPMENT CENTER ITALY SRL, viale Monte Grappa 3/5, 20124 Milano, Italia, Registro delle Imprese di Milano Monza Brianza Lodi REA n. 2504859, Capitale Sociale: 10.000 EUR i.v., Cod. Fisc. e P.IVA 10100050961, Societa con Socio Unico