[PATCH RT 24/25] sched: Provide migrate_disable/enable() inlines

[zanussi]

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

v4.14.170-rt75-rc1 stable review patch.
If anyone has any objections, please let me know.

-----------


[ Upstream commit 87d447be4100447b42229cce5e9b33c7915871eb ]

Currently code which solely needs to prevent migration of a task uses
preempt_disable()/enable() pairs. This is the only reliable way to do so as
setting the task affinity to a single CPU can be undone by a setaffinity
operation from a different task/process. It's also significantly faster.

RT provides a seperate migrate_disable/enable() mechanism which does not
disable preemption to achieve the semantic requirements of a (almost) fully
preemptible kernel.

As it is unclear from looking at a given code path whether the intention is
to disable preemption or migration, introduce migrate_disable/enable()
inline functions which can be used to annotate code which merely needs to
disable migration. Map them to preempt_disable/enable() for now. The RT
substitution will be provided later.

Code which is annotated that way documents that it has no requirement to
protect against reentrancy of a preempting task. Either this is not
required at all or the call sites are already serialized by other means.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Ben Segall <bsegall@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Signed-off-by: Tom Zanussi <zanussi@xxxxxxxxxx>
---
 include/linux/preempt.h | 26 ++++++++++++++++++++++++--
 1 file changed, 24 insertions(+), 2 deletions(-)

diff --git a/include/linux/preempt.h b/include/linux/preempt.h
index 6728662a81e8..2e15fbc01eda 100644
--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -241,8 +241,30 @@ static inline int __migrate_disabled(struct task_struct *p)
 }
 
 #else
-#define migrate_disable()		preempt_disable()
-#define migrate_enable()		preempt_enable()
+/**
+ * migrate_disable - Prevent migration of the current task
+ *
+ * Maps to preempt_disable() which also disables preemption. Use
+ * migrate_disable() to annotate that the intent is to prevent migration
+ * but not necessarily preemption.
+ *
+ * Can be invoked nested like preempt_disable() and needs the corresponding
+ * number of migrate_enable() invocations.
+ */
+#define migrate_disable()	preempt_disable()
+
+/**
+ * migrate_enable - Allow migration of the current task
+ *
+ * Counterpart to migrate_disable().
+ *
+ * As migrate_disable() can be invoked nested only the uttermost invocation
+ * reenables migration.
+ *
+ * Currently mapped to preempt_enable().
+ */
+#define migrate_enable()	preempt_enable()
+
 static inline int __migrate_disabled(struct task_struct *p)
 {
 	return 0;
-- 
2.14.1