sched: Further restrict the preemption modes

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
managable 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.

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>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Link: https://patch.msgid.link/20251219101502.GB1132199@noisy.programming.kicks-ass.net

1 files changed, 1 insertions, 1 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5b17d8e3cb55..fa720753e7d5 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7553,7 +7553,7 @@ int preempt_dynamic_mode = preempt_dynamic_undefined;
 
 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;