sched: Enable PREEMPT_DYNAMIC for PREEMPT_RT

In order to enable PREEMPT_DYNAMIC for PREEMPT_RT, remove PREEMPT_RT
from the 'Preemption Model' choice. Strictly speaking PREEMPT_RT is
not a change in how preemption works, but rather it makes a ton more
code preemptible.

Notably, take away NONE and VOLUNTARY options for PREEMPT_RT, they make
no sense (but are techincally possible).

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lkml.kernel.org/r/20241007075055.441622332@infradead.org

diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
index 09f06d8964cf0c33e67853b9a4f67a05c63911fd..7c1b29a3a491a15322974f36ce24efb9ec145d0a 100644 (file)
--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -20,6 +20,7 @@ choice
 
 config PREEMPT_NONE
        bool "No Forced Preemption (Server)"
+       depends on !PREEMPT_RT
        select PREEMPT_NONE_BUILD if !PREEMPT_DYNAMIC
        help
          This is the traditional Linux preemption model, geared towards
@@ -35,6 +36,7 @@ config PREEMPT_NONE
 config PREEMPT_VOLUNTARY
        bool "Voluntary Kernel Preemption (Desktop)"
        depends on !ARCH_NO_PREEMPT
+       depends on !PREEMPT_RT
        select PREEMPT_VOLUNTARY_BUILD if !PREEMPT_DYNAMIC
        help
          This option reduces the latency of the kernel by adding more
@@ -54,7 +56,7 @@ config PREEMPT_VOLUNTARY
 config PREEMPT
        bool "Preemptible Kernel (Low-Latency Desktop)"
        depends on !ARCH_NO_PREEMPT
-       select PREEMPT_BUILD
+       select PREEMPT_BUILD if !PREEMPT_DYNAMIC
        help
          This option reduces the latency of the kernel by making
          all kernel code (that is not executing in a critical section)
@@ -74,7 +76,7 @@ config PREEMPT_LAZY
        bool "Scheduler controlled preemption model"
        depends on !ARCH_NO_PREEMPT
        depends on ARCH_HAS_PREEMPT_LAZY
-       select PREEMPT_BUILD
+       select PREEMPT_BUILD if !PREEMPT_DYNAMIC
        help
          This option provides a scheduler driven preemption model that
          is fundamentally similar to full preemption, but is less
@@ -82,6 +84,8 @@ config PREEMPT_LAZY
          reduce lock holder preemption and recover some of the performance
          gains seen from using Voluntary preemption.
 
+endchoice
+
 config PREEMPT_RT
        bool "Fully Preemptible Kernel (Real-Time)"
        depends on EXPERT && ARCH_SUPPORTS_RT
@@ -99,8 +103,6 @@ config PREEMPT_RT
          Select this if you are building a kernel for systems which
          require real-time guarantees.
 
-endchoice
-
 config PREEMPT_COUNT
        bool
 
@@ -110,7 +112,7 @@ config PREEMPTION
 
 config PREEMPT_DYNAMIC
        bool "Preemption behaviour defined on boot"
-       depends on HAVE_PREEMPT_DYNAMIC && !PREEMPT_RT
+       depends on HAVE_PREEMPT_DYNAMIC
        select JUMP_LABEL if HAVE_PREEMPT_DYNAMIC_KEY
        select PREEMPT_BUILD
        default y if HAVE_PREEMPT_DYNAMIC_CALL

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index df6a34d27d2b448502cfe3c5815b538aa576e842..5c47d70f4204c01f67a7ee714fc5a9dce93f88a4 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7450,11 +7450,13 @@ int preempt_dynamic_mode = preempt_dynamic_undefined;
 
 int sched_dynamic_mode(const char *str)
 {
+#ifndef CONFIG_PREEMPT_RT
        if (!strcmp(str, "none"))
                return preempt_dynamic_none;
 
        if (!strcmp(str, "voluntary"))
                return preempt_dynamic_voluntary;
+#endif
 
        if (!strcmp(str, "full"))
                return preempt_dynamic_full;

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 44a49f90b05fee0c6b58ded36bc162c5dd2c96ee..a48b2a701ec2792d32e99d259dc69fdbd2724403 100644 (file)
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -248,9 +248,9 @@ static int sched_dynamic_show(struct seq_file *m, void *v)
                "none", "voluntary", "full", "lazy",
        };
        int j = ARRAY_SIZE(preempt_modes) - !IS_ENABLED(CONFIG_ARCH_HAS_PREEMPT_LAZY);
-       int i;
+       int i = IS_ENABLED(CONFIG_PREEMPT_RT) * 2;
 
-       for (i = 0; i < j; i++) {
+       for (; i < j; i++) {
                if (preempt_dynamic_mode == i)
                        seq_puts(m, "(");
                seq_puts(m, preempt_modes[i]);