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From: Valentin Schneider <valentin.schneider@arm.com>
Subject: sched: Introduce migratable()
Date: Wed, 11 Aug 2021 21:13:52 +0100
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/5.15/older/patches-5.15.3-rt21.tar.xz
Some areas use preempt_disable() + preempt_enable() to safely access
per-CPU data. The PREEMPT_RT folks have shown this can also be done by
keeping preemption enabled and instead disabling migration (and acquiring a
sleepable lock, if relevant).
Introduce a helper which checks whether the current task can be migrated
elsewhere, IOW if it is pinned to its local CPU in the current
context. This can help determining if per-CPU properties can be safely
accessed.
Note that CPU affinity is not checked here, as a preemptible task can have
its affinity changed at any given time (including if it has
PF_NO_SETAFFINITY, when hotplug gets involved).
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
[bigeasy: Return false on UP, call it is_migratable().]
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/20210811201354.1976839-3-valentin.schneider@arm.com
---
include/linux/sched.h | 10 ++++++++++
1 file changed, 10 insertions(+)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1730,6 +1730,16 @@ static __always_inline bool is_percpu_th
#endif
}
+/* Is the current task guaranteed to stay on its current CPU? */
+static inline bool is_migratable(void)
+{
+#ifdef CONFIG_SMP
+ return preemptible() && !current->migration_disabled;
+#else
+ return false;
+#endif
+}
+
/* Per-process atomic flags. */
#define PFA_NO_NEW_PRIVS 0 /* May not gain new privileges. */
#define PFA_SPREAD_PAGE 1 /* Spread page cache over cpuset */
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