[PATCH] sched: Further restrict the preemption modes

Peter Zijlstra
2025-12-19T10:15:08Z

[ with 6.18 being an LTS release, it might be a good time for this ]

The introduction of PREEMPT_LAZY was for multiple reasons:

- PREEMPT_RT suffered from over-scheduling, hurting performance compared to
!PREEMPT_RT.

- the introduction of (more) features that rely on preemption; like
folio_zero_user() which can do large memset() without preemption checks.

(Xen already had a horrible hack to deal with long running hypercalls)

- the endless and uncontrolled sprinkling of cond_resched() -- mostly cargo
cult or in response to poor to replicate workloads.

By moving to a model that is fundamentally preemptable these things become
manageable and avoid needing to introduce more horrible hacks.

Since this is a requirement; limit PREEMPT_NONE to architectures that do not
support preemption at all. Further limit PREEMPT_VOLUNTARY to those
architectures that do not yet have PREEMPT_LAZY support (with the eventual goal
to make this the empty set and completely remove voluntary preemption and
cond_resched() -- notably VOLUNTARY is already limited to !ARCH_NO_PREEMPT.)

This leaves up-to-date architectures (arm64, loongarch, powerpc, riscv, s390,
x86) with only two preemption models: full and lazy (like PREEMPT_RT).

While Lazy has been the recommended setting for a while, not all distributions
have managed to make the switch yet. Force things along. Keep the patch minimal
in case of hard to address regressions that might pop up.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/Kconfig.preempt | 3 +++
kernel/sched/core.c | 2 +-
kernel/sched/debug.c | 2 +-
3 files changed, 5 insertions(+), 2 deletions(-)

--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -16,11 +16,13 @@ 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
@@ -35,6 +37,7 @@ 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
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7553,7 +7553,7 @@ int preempt_dynamic_mode = preempt_dynam

int sched_dynamic_mode(const char *str)
{
-# ifndef CONFIG_PREEMPT_RT
+# if !(defined(CONFIG_PREEMPT_RT) || defined(CONFIG_ARCH_HAS_PREEMPT_LAZY))
if (!strcmp(str, "none"))
return preempt_dynamic_none;

--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -243,7 +243,7 @@ static ssize_t sched_dynamic_write(struc

static int sched_dynamic_show(struct seq_file *m, void *v)
{
- int i = IS_ENABLED(CONFIG_PREEMPT_RT) * 2;
+ int i = (IS_ENABLED(CONFIG_PREEMPT_RT) || IS_ENABLED(CONFIG_ARCH_HAS_PREEMPT_LAZY)) * 2;
int j;

/* Count entries in NULL terminated preempt_modes */