[PATCH] sched: Further restrict the preemption modes

[Peter Zijlstra <peterz@infradead.org>]

[ 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 */