rcu/nocb: Prepare for fine-grained deferred wakeup

Tuning the deferred wakeup level must be done from a safe wakeup
point. Currently those sites are:

* ->nocb_timer
* user/idle/guest entry
* CPU down
* softirq/rcuc

All of these sites perform the wake up for both RCU_NOCB_WAKE and
RCU_NOCB_WAKE_FORCE.

In order to merge ->nocb_timer and ->nocb_bypass_timer together, we plan
to add a new RCU_NOCB_WAKE_BYPASS that really should be deferred until
a timer fires so that we don't wake up the NOCB-gp kthread too early.

To prepare for that, this commit specifies the per-callsite wakeup
level/limit.

Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
[ paulmck: Fix non-NOCB rcu_nocb_need_deferred_wakeup() definition. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 8e78b24..5f1545a 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3911,7 +3911,7 @@ static int rcu_pending(int user)
        check_cpu_stall(rdp);
 
        /* Does this CPU need a deferred NOCB wakeup? */
-       if (rcu_nocb_need_deferred_wakeup(rdp))
+       if (rcu_nocb_need_deferred_wakeup(rdp, RCU_NOCB_WAKE))
                return 1;
 
        /* Is this a nohz_full CPU in userspace or idle?  (Ignore RCU if so.) */

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index b280a84..2510e86 100644 (file)
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -433,7 +433,7 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
                                bool *was_alldone, unsigned long flags);
 static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_empty,
                                 unsigned long flags);
-static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp);
+static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp, int level);
 static bool do_nocb_deferred_wakeup(struct rcu_data *rdp);
 static void rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp);
 static void rcu_spawn_cpu_nocb_kthread(int cpu);

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index db28e31..e2e5e49 100644 (file)
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2352,13 +2352,14 @@ static int rcu_nocb_cb_kthread(void *arg)
 }
 
 /* Is a deferred wakeup of rcu_nocb_kthread() required? */
-static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp)
+static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp, int level)
 {
-       return READ_ONCE(rdp->nocb_defer_wakeup) > RCU_NOCB_WAKE_NOT;
+       return READ_ONCE(rdp->nocb_defer_wakeup) >= level;
 }
 
 /* Do a deferred wakeup of rcu_nocb_kthread(). */
-static bool do_nocb_deferred_wakeup_common(struct rcu_data *rdp)
+static bool do_nocb_deferred_wakeup_common(struct rcu_data *rdp,
+                                          int level)
 {
        unsigned long flags;
        int ndw;
@@ -2367,7 +2368,7 @@ static bool do_nocb_deferred_wakeup_common(struct rcu_data *rdp)
 
        raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);
 
-       if (!rcu_nocb_need_deferred_wakeup(rdp_gp)) {
+       if (!rcu_nocb_need_deferred_wakeup(rdp_gp, level)) {
                raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);
                return false;
        }
@@ -2384,7 +2385,7 @@ static void do_nocb_deferred_wakeup_timer(struct timer_list *t)
 {
        struct rcu_data *rdp = from_timer(rdp, t, nocb_timer);
 
-       do_nocb_deferred_wakeup_common(rdp);
+       do_nocb_deferred_wakeup_common(rdp, RCU_NOCB_WAKE);
 }
 
 /*
@@ -2397,8 +2398,8 @@ static bool do_nocb_deferred_wakeup(struct rcu_data *rdp)
        if (!rdp->nocb_gp_rdp)
                return false;
 
-       if (rcu_nocb_need_deferred_wakeup(rdp->nocb_gp_rdp))
-               return do_nocb_deferred_wakeup_common(rdp);
+       if (rcu_nocb_need_deferred_wakeup(rdp->nocb_gp_rdp, RCU_NOCB_WAKE))
+               return do_nocb_deferred_wakeup_common(rdp, RCU_NOCB_WAKE);
        return false;
 }
 
@@ -2939,7 +2940,7 @@ static void __init rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp)
 {
 }
 
-static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp)
+static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp, int level)
 {
        return false;
 }