linux-kernel - [patch V3 06/64] sched: Provide schedule point for RT locks

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Date:   Thu, 05 Aug 2021 17:13:06 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Davidlohr Bueso <dave@...olabs.net>,
        Mike Galbraith <efault@....de>
Subject: [patch V3 06/64] sched: Provide schedule point for RT locks

From: Thomas Gleixner <tglx@...utronix.de>

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@...utronix.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
@@ -294,6 +294,9 @@ 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
@@ -5831,7 +5831,13 @@ 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.
@@ -6136,6 +6142,18 @@ 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 {