livepatch,sched: Add livepatch task switching to cond_resched()

There have been reports [1][2] of live patches failing to complete
within a reasonable amount of time due to CPU-bound kthreads.

Fix it by patching tasks in cond_resched().

There are four different flavors of cond_resched(), depending on the
kernel configuration.  Hook into all of them.

A more elegant solution might be to use a preempt notifier.  However,
non-ORC unwinders can't unwind a preempted task reliably.

[1] https://lore.kernel.org/lkml/20220507174628.2086373-1-song@kernel.org/
[2] https://lkml.kernel.org/lkml/20230120-vhost-klp-switching-v1-0-7c2b65519c43@kernel.org

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Tested-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
Link: https://lore.kernel.org/r/4ae981466b7814ec221014fc2554b2f86f3fb70b.1677257135.git.jpoimboe@kernel.org

1 files changed, 56 insertions, 8 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5ddd9610be56..b9616f153946 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8525,6 +8525,7 @@ EXPORT_STATIC_CALL_TRAMP(might_resched);
 static DEFINE_STATIC_KEY_FALSE(sk_dynamic_cond_resched);
 int __sched dynamic_cond_resched(void)
 {
+	klp_sched_try_switch();
 	if (!static_branch_unlikely(&sk_dynamic_cond_resched))
 		return 0;
 	return __cond_resched();
@@ -8673,13 +8674,17 @@ int sched_dynamic_mode(const char *str)
 #error "Unsupported PREEMPT_DYNAMIC mechanism"
 #endif
 
-void sched_dynamic_update(int mode)
+DEFINE_MUTEX(sched_dynamic_mutex);
+static bool klp_override;
+
+static void __sched_dynamic_update(int mode)
 {
 	/*
 	 * Avoid {NONE,VOLUNTARY} -> FULL transitions from ever ending up in
 	 * the ZERO state, which is invalid.
 	 */
-	preempt_dynamic_enable(cond_resched);
+	if (!klp_override)
+		preempt_dynamic_enable(cond_resched);
 	preempt_dynamic_enable(might_resched);
 	preempt_dynamic_enable(preempt_schedule);
 	preempt_dynamic_enable(preempt_schedule_notrace);
@@ -8687,36 +8692,79 @@ void sched_dynamic_update(int mode)
 
 	switch (mode) {
 	case preempt_dynamic_none:
-		preempt_dynamic_enable(cond_resched);
+		if (!klp_override)
+			preempt_dynamic_enable(cond_resched);
 		preempt_dynamic_disable(might_resched);
 		preempt_dynamic_disable(preempt_schedule);
 		preempt_dynamic_disable(preempt_schedule_notrace);
 		preempt_dynamic_disable(irqentry_exit_cond_resched);
-		pr_info("Dynamic Preempt: none\n");
+		if (mode != preempt_dynamic_mode)
+			pr_info("Dynamic Preempt: none\n");
 		break;
 
 	case preempt_dynamic_voluntary:
-		preempt_dynamic_enable(cond_resched);
+		if (!klp_override)
+			preempt_dynamic_enable(cond_resched);
 		preempt_dynamic_enable(might_resched);
 		preempt_dynamic_disable(preempt_schedule);
 		preempt_dynamic_disable(preempt_schedule_notrace);
 		preempt_dynamic_disable(irqentry_exit_cond_resched);
-		pr_info("Dynamic Preempt: voluntary\n");
+		if (mode != preempt_dynamic_mode)
+			pr_info("Dynamic Preempt: voluntary\n");
 		break;
 
 	case preempt_dynamic_full:
-		preempt_dynamic_disable(cond_resched);
+		if (!klp_override)
+			preempt_dynamic_disable(cond_resched);
 		preempt_dynamic_disable(might_resched);
 		preempt_dynamic_enable(preempt_schedule);
 		preempt_dynamic_enable(preempt_schedule_notrace);
 		preempt_dynamic_enable(irqentry_exit_cond_resched);
-		pr_info("Dynamic Preempt: full\n");
+		if (mode != preempt_dynamic_mode)
+			pr_info("Dynamic Preempt: full\n");
 		break;
 	}
 
 	preempt_dynamic_mode = mode;
 }
 
+void sched_dynamic_update(int mode)
+{
+	mutex_lock(&sched_dynamic_mutex);
+	__sched_dynamic_update(mode);
+	mutex_unlock(&sched_dynamic_mutex);
+}
+
+#ifdef CONFIG_HAVE_PREEMPT_DYNAMIC_CALL
+
+static int klp_cond_resched(void)
+{
+	__klp_sched_try_switch();
+	return __cond_resched();
+}
+
+void sched_dynamic_klp_enable(void)
+{
+	mutex_lock(&sched_dynamic_mutex);
+
+	klp_override = true;
+	static_call_update(cond_resched, klp_cond_resched);
+
+	mutex_unlock(&sched_dynamic_mutex);
+}
+
+void sched_dynamic_klp_disable(void)
+{
+	mutex_lock(&sched_dynamic_mutex);
+
+	klp_override = false;
+	__sched_dynamic_update(preempt_dynamic_mode);
+
+	mutex_unlock(&sched_dynamic_mutex);
+}
+
+#endif /* CONFIG_HAVE_PREEMPT_DYNAMIC_CALL */
+
 static int __init setup_preempt_mode(char *str)
 {
 	int mode = sched_dynamic_mode(str);