From 7dadeaa6e851e7d67733f3e24fc53ee107781d0f Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Thu, 18 Dec 2025 15:25:10 +0100 Subject: [PATCH] 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) Reviewed-by: Valentin Schneider Link: https://patch.msgid.link/20251219101502.GB1132199@noisy.programming.kicks-ass.net --- kernel/Kconfig.preempt | 3 +++ kernel/sched/core.c | 2 +- kernel/sched/debug.c | 2 +- 3 files changed, 5 insertions(+), 2 deletions(-) diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt index da326800c1c9b..88c594c6d7fcd 100644 --- 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 diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 5b17d8e3cb55c..fa720753e7d50 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; diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c index 41caa22e0680a..5f9b771951596 100644 --- a/kernel/sched/debug.c +++ b/kernel/sched/debug.c @@ -243,7 +243,7 @@ static ssize_t sched_dynamic_write(struct file *filp, const char __user *ubuf, 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 */ -- 2.47.3