[patch V4 06/68] sched: Provide schedule point for RT locks

Thomas Gleixner
2021-08-11T12:23:26Z

RT enabled kernels substitute spin/rwlocks with 'sleeping' variants based
on rtmutex. Blocking on such a lock is similar to preemption versus:

- I/O scheduling and worker handling because these functions might block
on another substituted lock or come from a lock contention within these
functions.

- RCU considers this like a preemption because the task might be in a read
side critical section.

Add a separate scheduling point for this and hand a new scheduling mode
argument to __schedule() which allows along with separate mode masks to
handle this gracefully from within the scheduler without proliferating that
to other subsystems like RCU.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
V2: Adopt to the simplified mask logic
---
include/linux/sched.h | 3 +++
kernel/sched/core.c | 20 +++++++++++++++++++-
2 files changed, 22 insertions(+), 1 deletion(-)
---
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -288,6 +288,9 @@ extern long schedule_timeout_idle(long t
asmlinkage void schedule(void);
extern void schedule_preempt_disabled(void);
asmlinkage void preempt_schedule_irq(void);
+#ifdef CONFIG_PREEMPT_RT
+ extern void schedule_rtlock(void);
+#endif

extern int __must_check io_schedule_prepare(void);
extern void io_schedule_finish(int token);
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5829,7 +5829,13 @@ pick_next_task(struct rq *rq, struct tas
*/
#define SM_NONE 0x0
#define SM_PREEMPT 0x1
-#define SM_MASK_PREEMPT (~0U)
+#define SM_RTLOCK_WAIT 0x2
+
+#ifndef CONFIG_PREEMPT_RT
+# define SM_MASK_PREEMPT (~0U)
+#else
+# define SM_MASK_PREEMPT SM_PREEMPT
+#endif

/*
* __schedule() is the main scheduler function.
@@ -6134,6 +6140,18 @@ void __sched schedule_preempt_disabled(v
preempt_disable();
}

+#ifdef CONFIG_PREEMPT_RT
+void __sched notrace schedule_rtlock(void)
+{
+ do {
+ preempt_disable();
+ __schedule(SM_RTLOCK_WAIT);
+ sched_preempt_enable_no_resched();
+ } while (need_resched());
+}
+NOKPROBE_SYMBOL(schedule_rtlock);
+#endif
+
static void __sched notrace preempt_schedule_common(void)
{
do {