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

From: zanussi@kernel.org
To: LKML <linux-kernel@vger.kernel.org>,
	linux-rt-users <linux-rt-users@vger.kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Carsten Emde <C.Emde@osadl.org>, John Kacur <jkacur@redhat.com>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Daniel Wagner <wagi@monom.org>, Tom Zanussi <zanussi@kernel.org>
Subject: [PATCH RT 24/25] sched: Provide migrate_disable/enable() inlines
Date: Fri, 21 Feb 2020 15:24:52 -0600	[thread overview]
Message-ID: <5e82e4f7f3bc60945e64b2ee8ac429d6c5b51838.1582320278.git.zanussi@kernel.org> (raw)
In-Reply-To: <cover.1582320278.git.zanussi@kernel.org>
In-Reply-To: <cover.1582320278.git.zanussi@kernel.org>

From: Thomas Gleixner <tglx@linutronix.de>

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@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Tom Zanussi <zanussi@kernel.org>
---
 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

