Re: [RFC PATCH 01/12] rcu: Implement rcu_segcblist_is_offloaded() config dependent

["Paul E. McKenney" <paulmck@kernel.org>]

On Tue, Sep 22, 2020 at 11:43:26PM +0200, Frederic Weisbecker wrote:
> On Mon, Sep 21, 2020 at 05:27:32PM -0700, Paul E. McKenney wrote:
> > On Mon, Sep 21, 2020 at 02:43:40PM +0200, Frederic Weisbecker wrote:
> > > This simplify the usage of this API and avoid checking the kernel
> > > config from the callers.
> > > 
> > > Suggested-by: Paul E. McKenney <paulmck@kernel.org>
> > > Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> > > Cc: Paul E. McKenney <paulmck@kernel.org>
> > > Cc: Josh Triplett <josh@joshtriplett.org>
> > > Cc: Steven Rostedt <rostedt@goodmis.org>
> > > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > > Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> > > Cc: Joel Fernandes <joel@joelfernandes.org>
> > 
> > Nice cleanup!  I clearly should have done it this way to start with.
> > 
> > Any reason I shouldn't pull this into -rcu right now?
> 
> I think that very one can be applied indeed.

Very well!  I had to hand-apply it due to recent -rcu thrash, and as
usual I could not resist wordsmithing the commit log.  Please let
me know if I messed anything up.

							Thanx, Paul

------------------------------------------------------------------------

commit 99a2a9b00b1f44019e5667cdbd44b8eb82e9f0a9
Author: Frederic Weisbecker <frederic@kernel.org>
Date:   Mon Sep 21 14:43:40 2020 +0200

    rcu: Implement rcu_segcblist_is_offloaded() config dependent
    
    This commit simplifies the use of the rcu_segcblist_is_offloaded() API so
    that its callers no longer need to check the RCU_NOCB_CPU Kconfig option.
    Note that rcu_segcblist_is_offloaded() is defined in the header file,
    which means that the generated code should be just as efficient as before.
    
    Suggested-by: Paul E. McKenney <paulmck@kernel.org>
    Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
    Cc: Paul E. McKenney <paulmck@kernel.org>
    Cc: Josh Triplett <josh@joshtriplett.org>
    Cc: Steven Rostedt <rostedt@goodmis.org>
    Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
    Cc: Lai Jiangshan <jiangshanlai@gmail.com>
    Cc: Joel Fernandes <joel@joelfernandes.org>
    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h
index 5c293af..492262b 100644
--- a/kernel/rcu/rcu_segcblist.h
+++ b/kernel/rcu/rcu_segcblist.h
@@ -62,7 +62,7 @@ static inline bool rcu_segcblist_is_enabled(struct rcu_segcblist *rsclp)
 /* Is the specified rcu_segcblist offloaded?  */
 static inline bool rcu_segcblist_is_offloaded(struct rcu_segcblist *rsclp)
 {
-	return rsclp->offloaded;
+	return IS_ENABLED(CONFIG_RCU_NOCB_CPU) && rsclp->offloaded;
 }
 
 /*
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 2424e2a..6d9ec8e 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1609,8 +1609,7 @@ static bool __note_gp_changes(struct rcu_node *rnp, struct rcu_data *rdp)
 {
 	bool ret = false;
 	bool need_qs;
-	const bool offloaded = IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
-			       rcu_segcblist_is_offloaded(&rdp->cblist);
+	const bool offloaded = rcu_segcblist_is_offloaded(&rdp->cblist);
 
 	raw_lockdep_assert_held_rcu_node(rnp);
 
@@ -2054,8 +2053,7 @@ static void rcu_gp_cleanup(void)
 		needgp = true;
 	}
 	/* Advance CBs to reduce false positives below. */
-	offloaded = IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
-		    rcu_segcblist_is_offloaded(&rdp->cblist);
+	offloaded = rcu_segcblist_is_offloaded(&rdp->cblist);
 	if ((offloaded || !rcu_accelerate_cbs(rnp, rdp)) && needgp) {
 		WRITE_ONCE(rcu_state.gp_flags, RCU_GP_FLAG_INIT);
 		WRITE_ONCE(rcu_state.gp_req_activity, jiffies);
@@ -2254,8 +2252,7 @@ rcu_report_qs_rdp(struct rcu_data *rdp)
 	unsigned long flags;
 	unsigned long mask;
 	bool needwake = false;
-	const bool offloaded = IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
-			       rcu_segcblist_is_offloaded(&rdp->cblist);
+	const bool offloaded = rcu_segcblist_is_offloaded(&rdp->cblist);
 	struct rcu_node *rnp;
 
 	WARN_ON_ONCE(rdp->cpu != smp_processor_id());
@@ -2423,8 +2420,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
 {
 	int div;
 	unsigned long flags;
-	const bool offloaded = IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
-			       rcu_segcblist_is_offloaded(&rdp->cblist);
+	const bool offloaded = rcu_segcblist_is_offloaded(&rdp->cblist);
 	struct rcu_head *rhp;
 	struct rcu_cblist rcl = RCU_CBLIST_INITIALIZER(rcl);
 	long bl, count;
@@ -2681,8 +2677,7 @@ static __latent_entropy void rcu_core(void)
 	unsigned long flags;
 	struct rcu_data *rdp = raw_cpu_ptr(&rcu_data);
 	struct rcu_node *rnp = rdp->mynode;
-	const bool offloaded = IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
-			       rcu_segcblist_is_offloaded(&rdp->cblist);
+	const bool offloaded = rcu_segcblist_is_offloaded(&rdp->cblist);
 
 	if (cpu_is_offline(smp_processor_id()))
 		return;
@@ -2984,8 +2979,7 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func)
 				   rcu_segcblist_n_cbs(&rdp->cblist));
 
 	/* Go handle any RCU core processing required. */
-	if (IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
-	    unlikely(rcu_segcblist_is_offloaded(&rdp->cblist))) {
+	if (unlikely(rcu_segcblist_is_offloaded(&rdp->cblist))) {
 		__call_rcu_nocb_wake(rdp, was_alldone, flags); /* unlocks */
 	} else {
 		__call_rcu_core(rdp, head, flags);
@@ -3739,8 +3733,7 @@ static int rcu_pending(int user)
 
 	/* Has RCU gone idle with this CPU needing another grace period? */
 	if (!gp_in_progress && rcu_segcblist_is_enabled(&rdp->cblist) &&
-	    (!IS_ENABLED(CONFIG_RCU_NOCB_CPU) ||
-	     !rcu_segcblist_is_offloaded(&rdp->cblist)) &&
+	    !rcu_segcblist_is_offloaded(&rdp->cblist) &&
 	    !rcu_segcblist_restempty(&rdp->cblist, RCU_NEXT_READY_TAIL))
 		return 1;