From 4ac8227a3c744372d9f8eaf801c0d8e3be0b70aa Mon Sep 17 00:00:00 2001
From: "Paul E. McKenney" <paulmck@kernel.org>
Date: Tue, 9 Nov 2021 13:37:34 -0800
Subject: [PATCH] rcu-tasks: Abstract checking of callback lists

References: bnc#1189998 (PREEMPT_RT prerequisite backports)
Patch-mainline: v5.17-rc0
Git-commit: 4d1114c05467b5f421d99121bff22a9633390722

This commit adds a rcu_tasks_need_gpcb() function that returns an
indication of whether another grace period is required, and if no grace
period is required, whether there are callbacks that need to be invoked.
The function scans all per-CPU lists currently in use.

Reported-by: Martin Lau <kafai@fb.com>
Cc: Neeraj Upadhyay <neeraj.iitr10@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 kernel/rcu/tasks.h | 61 ++++++++++++++++++++++++++++++++++--------------------
 1 file changed, 38 insertions(+), 23 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 03c44c9b3909..47ac74ea8d9c 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -229,11 +229,38 @@ static void synchronize_rcu_tasks_generic(struct rcu_tasks *rtp)
 	wait_rcu_gp(rtp->call_func);
 }
 
+// Advance callbacks and indicate whether either a grace period or
+// callback invocation is needed.
+static int rcu_tasks_need_gpcb(struct rcu_tasks *rtp)
+{
+	int cpu;
+	unsigned long flags;
+	int needgpcb = 0;
+
+	for (cpu = 0; cpu < rtp->percpu_enqueue_lim; cpu++) {
+		struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
+
+		/* Advance and accelerate any new callbacks. */
+		if (rcu_segcblist_empty(&rtpcp->cblist))
+			continue;
+		raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
+		rcu_segcblist_advance(&rtpcp->cblist, rcu_seq_current(&rtp->tasks_gp_seq));
+		(void)rcu_segcblist_accelerate(&rtpcp->cblist, rcu_seq_snap(&rtp->tasks_gp_seq));
+		if (rcu_segcblist_pend_cbs(&rtpcp->cblist))
+			needgpcb |= 0x3;
+		if (!rcu_segcblist_empty(&rtpcp->cblist))
+			needgpcb |= 0x1;
+		raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
+	}
+	return needgpcb;
+}
+
 /* RCU-tasks kthread that detects grace periods and invokes callbacks. */
 static int __noreturn rcu_tasks_kthread(void *arg)
 {
 	unsigned long flags;
 	int len;
+	int needgpcb;
 	struct rcu_cblist rcl = RCU_CBLIST_INITIALIZER(rcl);
 	struct rcu_head *rhp;
 	struct rcu_tasks *rtp = arg;
@@ -249,37 +276,25 @@ static int __noreturn rcu_tasks_kthread(void *arg)
 	 * This loop is terminated by the system going down.  ;-)
 	 */
 	for (;;) {
-		struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, 0);  // for_each...
+		struct rcu_tasks_percpu *rtpcp;
 
 		set_tasks_gp_state(rtp, RTGS_WAIT_CBS);
 
-		/* Pick up any new callbacks. */
-		raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
-		rcu_segcblist_advance(&rtpcp->cblist, rcu_seq_current(&rtp->tasks_gp_seq));
-		(void)rcu_segcblist_accelerate(&rtpcp->cblist, rcu_seq_snap(&rtp->tasks_gp_seq));
-		raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
-
 		/* If there were none, wait a bit and start over. */
-		if (!rcu_segcblist_pend_cbs(&rtpcp->cblist)) {
-			wait_event_interruptible(rtp->cbs_wq,
-						 rcu_segcblist_pend_cbs(&rtpcp->cblist));
-			if (!rcu_segcblist_pend_cbs(&rtpcp->cblist)) {
-				WARN_ON(signal_pending(current));
-				set_tasks_gp_state(rtp, RTGS_WAIT_WAIT_CBS);
-				schedule_timeout_idle(HZ/10);
-			}
-			continue;
+		wait_event_idle(rtp->cbs_wq, (needgpcb = rcu_tasks_need_gpcb(rtp)));
+
+		if (needgpcb & 0x2) {
+			// Wait for one grace period.
+			set_tasks_gp_state(rtp, RTGS_WAIT_GP);
+			rtp->gp_start = jiffies;
+			rcu_seq_start(&rtp->tasks_gp_seq);
+			rtp->gp_func(rtp);
+			rcu_seq_end(&rtp->tasks_gp_seq);
 		}
 
-		// Wait for one grace period.
-		set_tasks_gp_state(rtp, RTGS_WAIT_GP);
-		rtp->gp_start = jiffies;
-		rcu_seq_start(&rtp->tasks_gp_seq);
-		rtp->gp_func(rtp);
-		rcu_seq_end(&rtp->tasks_gp_seq);
-
 		/* Invoke the callbacks. */
 		set_tasks_gp_state(rtp, RTGS_INVOKE_CBS);
+		rtpcp = per_cpu_ptr(rtp->rtpcpu, 0);
 		raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
 		rcu_segcblist_advance(&rtpcp->cblist, rcu_seq_current(&rtp->tasks_gp_seq));
 		rcu_segcblist_extract_done_cbs(&rtpcp->cblist, &rcl);