From 220a87a5fad8603977dae8c2da8480b0a26b611e Mon Sep 17 00:00:00 2001
From: "Paul E. McKenney" <paulmck@kernel.org>
Date: Tue, 9 Nov 2021 15:34:56 -0800
Subject: [PATCH] rcu-tasks: Abstract invocations of callbacks

References: bnc#1189998 (PREEMPT_RT prerequisite backports)
Patch-mainline: v5.17-rc0
Git-commit: 57881863ad15fbccbfa637b5e4b67cd3a4520643

This commit adds a rcu_tasks_invoke_cbs() function that invokes all
ready callbacks on all of the per-CPU lists that are 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 | 56 +++++++++++++++++++++++++++++++++---------------------
 1 file changed, 34 insertions(+), 22 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 47ac74ea8d9c..5ab3509238a2 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -255,14 +255,42 @@ static int rcu_tasks_need_gpcb(struct rcu_tasks *rtp)
 	return needgpcb;
 }
 
-/* RCU-tasks kthread that detects grace periods and invokes callbacks. */
-static int __noreturn rcu_tasks_kthread(void *arg)
+// Advance callbacks and invoke any that are ready.
+static void rcu_tasks_invoke_cbs(struct rcu_tasks *rtp)
 {
+	int cpu;
 	unsigned long flags;
 	int len;
-	int needgpcb;
 	struct rcu_cblist rcl = RCU_CBLIST_INITIALIZER(rcl);
 	struct rcu_head *rhp;
+
+	for (cpu = 0; cpu < rtp->percpu_enqueue_lim; cpu++) {
+		struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
+
+		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));
+		rcu_segcblist_extract_done_cbs(&rtpcp->cblist, &rcl);
+		raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
+		len = rcl.len;
+		for (rhp = rcu_cblist_dequeue(&rcl); rhp; rhp = rcu_cblist_dequeue(&rcl)) {
+			local_bh_disable();
+			rhp->func(rhp);
+			local_bh_enable();
+			cond_resched();
+		}
+		raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
+		rcu_segcblist_add_len(&rtpcp->cblist, -len);
+		(void)rcu_segcblist_accelerate(&rtpcp->cblist, rcu_seq_snap(&rtp->tasks_gp_seq));
+		raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
+	}
+}
+
+/* RCU-tasks kthread that detects grace periods and invokes callbacks. */
+static int __noreturn rcu_tasks_kthread(void *arg)
+{
+	int needgpcb;
 	struct rcu_tasks *rtp = arg;
 
 	/* Run on housekeeping CPUs by default.  Sysadm can move if desired. */
@@ -276,8 +304,6 @@ static int __noreturn rcu_tasks_kthread(void *arg)
 	 * This loop is terminated by the system going down.  ;-)
 	 */
 	for (;;) {
-		struct rcu_tasks_percpu *rtpcp;
-
 		set_tasks_gp_state(rtp, RTGS_WAIT_CBS);
 
 		/* If there were none, wait a bit and start over. */
@@ -292,24 +318,10 @@ static int __noreturn rcu_tasks_kthread(void *arg)
 			rcu_seq_end(&rtp->tasks_gp_seq);
 		}
 
-		/* Invoke the callbacks. */
+		/* Invoke 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);
-		raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
-		len = rcl.len;
-		for (rhp = rcu_cblist_dequeue(&rcl); rhp; rhp = rcu_cblist_dequeue(&rcl)) {
-			local_bh_disable();
-			rhp->func(rhp);
-			local_bh_enable();
-			cond_resched();
-		}
-		raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
-		rcu_segcblist_add_len(&rtpcp->cblist, -len);
-		(void)rcu_segcblist_accelerate(&rtpcp->cblist, rcu_seq_snap(&rtp->tasks_gp_seq));
-		raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
+		rcu_tasks_invoke_cbs(rtp);
+
 		/* Paranoid sleep to keep this from entering a tight loop */
 		schedule_timeout_idle(rtp->gp_sleep);
 	}