diff options
| author | Peter Zijlstra <peterz@infradead.org> | 2025-12-18 15:25:10 +0100 |
|---|---|---|
| committer | Peter Zijlstra <peterz@infradead.org> | 2026-01-08 12:43:57 +0100 |
| commit | 7dadeaa6e851e7d67733f3e24fc53ee107781d0f (patch) | |
| tree | 22065c19a3f531f3168501c61ba3b0857c3ed64e /kernel/sched | |
| parent | 89951fc1f8201df27366ac1eed1ddc9ee0f47729 (diff) | |
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
Diffstat (limited to 'kernel/sched')
| -rw-r--r-- | kernel/sched/core.c | 2 | ||||
| -rw-r--r-- | kernel/sched/debug.c | 2 |
2 files changed, 2 insertions, 2 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; diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c index 41caa22e0680..5f9b77195159 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 */ |