linux-kernel - [PATCH tip/core/rcu 09/21] rcu: Make rcu_migrate

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Date:   Sun, 22 Apr 2018 20:03:32 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     linux-kernel@...r.kernel.org
Cc:     mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel.opensrc@...il.com,
        torvalds@...ux-foundation.org, npiggin@...il.com,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 09/21] rcu: Make rcu_migrate_callbacks wake GP kthread when needed

The rcu_migrate_callbacks() function invokes rcu_advance_cbs()
twice, ignoring the return value.  This is OK at pressent because of
failsafe code that does the wakeup when needed.  However, this failsafe
code acquires the root rcu_node structure's lock frequently, while
rcu_migrate_callbacks() does so only once per CPU-offline operation.

This commit therefore makes rcu_migrate_callbacks()
wake up the RCU GP kthread when either call to rcu_advance_cbs()
returns true, thus removing need for the failsafe code.

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 6ef1f2b4a6d3..f75eb5174021 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3876,6 +3876,7 @@ static void rcu_migrate_callbacks(int cpu, struct rcu_state *rsp)
 	struct rcu_data *my_rdp;
 	struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
 	struct rcu_node *rnp_root = rcu_get_root(rdp->rsp);
+	bool needwake;
 
 	if (rcu_is_nocb_cpu(cpu) || rcu_segcblist_empty(&rdp->cblist))
 		return;  /* No callbacks to migrate. */
@@ -3887,12 +3888,15 @@ static void rcu_migrate_callbacks(int cpu, struct rcu_state *rsp)
 		return;
 	}
 	raw_spin_lock_rcu_node(rnp_root); /* irqs already disabled. */
-	rcu_advance_cbs(rsp, rnp_root, rdp); /* Leverage recent GPs. */
-	rcu_advance_cbs(rsp, rnp_root, my_rdp); /* Assign GP to pending CBs. */
+	/* Leverage recent GPs and set GP for new callbacks. */
+	needwake = rcu_advance_cbs(rsp, rnp_root, rdp) ||
+		   rcu_advance_cbs(rsp, rnp_root, my_rdp);
 	rcu_segcblist_merge(&my_rdp->cblist, &rdp->cblist);
 	WARN_ON_ONCE(rcu_segcblist_empty(&my_rdp->cblist) !=
 		     !rcu_segcblist_n_cbs(&my_rdp->cblist));
 	raw_spin_unlock_irqrestore_rcu_node(rnp_root, flags);
+	if (needwake)
+		rcu_gp_kthread_wake(rsp);
 	WARN_ONCE(rcu_segcblist_n_cbs(&rdp->cblist) != 0 ||
 		  !rcu_segcblist_empty(&rdp->cblist),
 		  "rcu_cleanup_dead_cpu: Callbacks on offline CPU %d: qlen=%lu, 1stCB=%p\n",
-- 
2.5.2