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sched: Further restrict the preemption modes

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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
This commit is contained in:
Peter Zijlstra 2025-12-18 15:25:10 +01:00
parent 89951fc1f8
commit 7dadeaa6e8
3 changed files with 5 additions and 2 deletions
Download patch file Download diff file Expand all files Collapse all files

3
kernel/Kconfig.preempt
View file

@ -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

2
kernel/sched/core.c
View file

@ -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;

2
kernel/sched/debug.c
View file

@ -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 */