[patch 05/50] sched: Provide schedule point for RT locks

From: Thomas Gleixner <tglx@linutronix.de>
To: LKML <linux-kernel@vger.kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Will Deacon <will@kernel.org>, Waiman Long <longman@redhat.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Davidlohr Bueso <dave@stgolabs.net>
Subject: [patch 05/50] sched: Provide schedule point for RT locks
Date: Tue, 13 Jul 2021 17:10:59 +0200	[thread overview]
Message-ID: <20210713160746.410991567@linutronix.de> (raw)
In-Reply-To: 20210713151054.700719949@linutronix.de

From: Thomas Gleixner <tglx@linutronix.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 seperate scheduling point for this and hand a new scheduling mode
argument to __schedule() which allows along with seperate 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>
---
 include/linux/sched.h |    3 +++
 kernel/sched/core.c   |   22 ++++++++++++++++++++--
 2 files changed, 23 insertions(+), 2 deletions(-)
---

--- 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
@@ -5832,8 +5832,14 @@ pick_next_task(struct rq *rq, struct tas
  */
 #define	SM_NONE			0x0
 #define	SM_PREEMPT		0x1
-#define SM_MASK_PREEMPT		UINT_MAX
-#define SM_MASK_STATE		SM_MASK_PREEMPT
+#ifndef CONFIG_PREEMPT_RT
+# define SM_MASK_PREEMPT	UINT_MAX
+# define SM_MASK_STATE		SM_MASK_PREEMPT
+#else
+# define SM_RTLOCK_WAIT		0x2
+# define SM_MASK_PREEMPT	SM_PREEMPT
+# define SM_MASK_STATE		(SM_PREEMPT | SM_RTLOCK_WAIT)
+#endif
 
 /*
  * __schedule() is the main scheduler function.
@@ -6138,6 +6144,18 @@ 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 {

