Commit ec4eacce authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

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: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: default avatarNicholas Piggin <npiggin@gmail.com>
parent 6f576e28

kernel/rcu/tree.c

+6 −2
Original line number Diff line number Diff line
@@ -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",

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