sched: Provide migrate_disable/enable() inlines

sched: Provide migrate_disable/enable() inlines

[Thomas Gleixner]

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.

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>
---
 include/linux/preempt.h |   30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -322,4 +322,34 @@ static inline void preempt_notifier_init
 
 #endif
 
+/**
+ * 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.
+ */
+static __always_inline void migrate_disable(void)
+{
+	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().
+ */
+static __always_inline void migrate_enable(void)
+{
+	preempt_enable();
+}
+
 #endif /* __LINUX_PREEMPT_H */

Re: sched: Provide migrate_disable/enable() inlines

[Ingo Molnar]

* Thomas Gleixner <tglx@linutronix.de> wrote:

> 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.
> 
> 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>
> ---
>  include/linux/preempt.h |   30 ++++++++++++++++++++++++++++++
>  1 file changed, 30 insertions(+)
> 
> --- a/include/linux/preempt.h
> +++ b/include/linux/preempt.h
> @@ -322,4 +322,34 @@ static inline void preempt_notifier_init
>  
>  #endif
>  
> +/**
> + * 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.
> + */
> +static __always_inline void migrate_disable(void)
> +{
> +	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().
> + */
> +static __always_inline void migrate_enable(void)
> +{
> +	preempt_enable();
> +}

I've applied this with s/uttermost/outermost, which I suspect was the 
intention?

Thanks,

	Ingo

[tip: sched/core] sched/rt: Provide migrate_disable/enable() inlines

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     9fe65747dbacf0c9fcf12350c3afb5fcf7e78534
Gitweb:        https://git.kernel.org/tip/9fe65747dbacf0c9fcf12350c3afb5fcf7e78534
Author:        Thomas Gleixner <tglx@linutronix.de>
AuthorDate:    Sat, 08 Feb 2020 20:48:29 +01:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Sun, 09 Feb 2020 20:07:41 +01:00

sched/rt: Provide migrate_disable/enable() inlines

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.

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>
Signed-off-by: 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>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/878slclv1u.fsf@nanos.tec.linutronix.de
---
 include/linux/preempt.h | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/include/linux/preempt.h b/include/linux/preempt.h
index bbb68db..bc3f1ae 100644
--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -322,4 +322,34 @@ static inline void preempt_notifier_init(struct preempt_notifier *notifier,
 
 #endif
 
+/**
+ * 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.
+ */
+static __always_inline void migrate_disable(void)
+{
+	preempt_disable();
+}
+
+/**
+ * migrate_enable - Allow migration of the current task
+ *
+ * Counterpart to migrate_disable().
+ *
+ * As migrate_disable() can be invoked nested, only the outermost invocation
+ * reenables migration.
+ *
+ * Currently mapped to preempt_enable().
+ */
+static __always_inline void migrate_enable(void)
+{
+	preempt_enable();
+}
+
 #endif /* __LINUX_PREEMPT_H */

[tip: sched/rt] sched/rt: Provide migrate_disable/enable() inlines

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the sched/rt branch of tip:

Commit-ID:     66630058e56b26b3a9cf2625e250a8c592dd0207
Gitweb:        https://git.kernel.org/tip/66630058e56b26b3a9cf2625e250a8c592dd0207
Author:        Thomas Gleixner <tglx@linutronix.de>
AuthorDate:    Sat, 08 Feb 2020 20:48:29 +01:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Thu, 20 Feb 2020 21:17:24 +01:00

sched/rt: Provide migrate_disable/enable() inlines

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.

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>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
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>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/878slclv1u.fsf@nanos.tec.linutronix.de

---
 include/linux/preempt.h | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/include/linux/preempt.h b/include/linux/preempt.h
index bbb68db..bc3f1ae 100644
--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -322,4 +322,34 @@ static inline void preempt_notifier_init(struct preempt_notifier *notifier,
 
 #endif
 
+/**
+ * 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.
+ */
+static __always_inline void migrate_disable(void)
+{
+	preempt_disable();
+}
+
+/**
+ * migrate_enable - Allow migration of the current task
+ *
+ * Counterpart to migrate_disable().
+ *
+ * As migrate_disable() can be invoked nested, only the outermost invocation
+ * reenables migration.
+ *
+ * Currently mapped to preempt_enable().
+ */
+static __always_inline void migrate_enable(void)
+{
+	preempt_enable();
+}
+
 #endif /* __LINUX_PREEMPT_H */