Commit 7dadeaa6 authored by Peter Zijlstra's avatar Peter Zijlstra
Browse files

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: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarValentin Schneider <vschneid@redhat.com>
Link: https://patch.msgid.link/20251219101502.GB1132199@noisy.programming.kicks-ass.net
parent 89951fc1

kernel/Kconfig.preempt

+3 −0
Original line number Diff line number Diff line
@@ -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

kernel/sched/core.c

+1 −1
Original line number Diff line number Diff line
@@ -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;

kernel/sched/debug.c

+1 −1
Original line number Diff line number Diff line
@@ -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 */