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

[Salvatore Dipietro <dipiets@amazon.it>]

Commit 7dadeaa6e851 ("sched: Further restrict the preemption
modes") changed the default preemption model to PREEMPT_LAZY on
architectures that support it and made PREEMPT_NONE depend on
ARCH_NO_PREEMPT.

This causes a 0.51x throughput regression on PostgreSQL pgbench
(simple-update) with 1024 clients on a 96-vCPU Graviton4 arm64
system. Perf profiling shows 55% of CPU time spinning in
PostgreSQL's userspace spinlock (s_lock()) under PREEMPT_LAZY.

Restore PREEMPT_NONE as the default preemption model, remove
the ARCH_NO_PREEMPT dependency from PREEMPT_NONE, and remove
the ARCH_HAS_PREEMPT_LAZY restriction from PREEMPT_VOLUNTARY.

Fixes: 7dadeaa6e851 ("sched: Further restrict the preemption modes")
Signed-off-by: Salvatore Dipietro <dipiets@amazon.it>
---
 kernel/Kconfig.preempt | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
index 88c594c6d7fc..da326800c1c9 100644
--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -16,13 +16,11 @@ config ARCH_HAS_PREEMPT_LAZY
 
 choice
 	prompt "Preemption Model"
-	default PREEMPT_LAZY if ARCH_HAS_PREEMPT_LAZY
 	default PREEMPT_NONE
 
 config PREEMPT_NONE
 	bool "No Forced Preemption (Server)"
 	depends on !PREEMPT_RT
-	depends on ARCH_NO_PREEMPT
 	select PREEMPT_NONE_BUILD if !PREEMPT_DYNAMIC
 	help
 	  This is the traditional Linux preemption model, geared towards
@@ -37,7 +35,6 @@ config PREEMPT_NONE
 
 config PREEMPT_VOLUNTARY
 	bool "Voluntary Kernel Preemption (Desktop)"
-	depends on !ARCH_HAS_PREEMPT_LAZY
 	depends on !ARCH_NO_PREEMPT
 	depends on !PREEMPT_RT
 	select PREEMPT_VOLUNTARY_BUILD if !PREEMPT_DYNAMIC
-- 
2.50.1 (Apple Git-155)




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