rcu: Set a maximum limit for back-to-back callback invocation

Currently, if a CPU has more than 10,000 callbacks pending, it will
increase rdp->blimit to LONG_MAX.  If you are lucky, LONG_MAX is only
about two billion, but this is still a bit too many callbacks to invoke
back-to-back while otherwise ignoring the world.

This commit therefore sets a maximum limit of DEFAULT_MAX_RCU_BLIMIT,
which is set to 10,000, for rdp->blimit.

Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 980ca3ca643fbf09c35b4e1b7810d196be511f23..f888a76673da08702da3d45ce5a4d15fe60c6f31 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -380,7 +380,8 @@ static int rcu_is_cpu_rrupt_from_idle(void)
               __this_cpu_read(rcu_data.dynticks_nmi_nesting) <= 1;
 }
 
-#define DEFAULT_RCU_BLIMIT 10     /* Maximum callbacks per rcu_do_batch. */
+#define DEFAULT_RCU_BLIMIT 10     /* Maximum callbacks per rcu_do_batch ... */
+#define DEFAULT_MAX_RCU_BLIMIT 10000 /* ... even during callback flood. */
 static long blimit = DEFAULT_RCU_BLIMIT;
 #define DEFAULT_RCU_QHIMARK 10000 /* If this many pending, ignore blimit. */
 static long qhimark = DEFAULT_RCU_QHIMARK;
@@ -2113,7 +2114,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
 
        /* Reinstate batch limit if we have worked down the excess. */
        count = rcu_segcblist_n_cbs(&rdp->cblist);
-       if (rdp->blimit == LONG_MAX && count <= qlowmark)
+       if (rdp->blimit >= DEFAULT_MAX_RCU_BLIMIT && count <= qlowmark)
                rdp->blimit = blimit;
 
        /* Reset ->qlen_last_fqs_check trigger if enough CBs have drained. */
@@ -2354,7 +2355,7 @@ static void __call_rcu_core(struct rcu_data *rdp, struct rcu_head *head,
                        rcu_accelerate_cbs_unlocked(rdp->mynode, rdp);
                } else {
                        /* Give the grace period a kick. */
-                       rdp->blimit = LONG_MAX;
+                       rdp->blimit = DEFAULT_MAX_RCU_BLIMIT;
                        if (rcu_state.n_force_qs == rdp->n_force_qs_snap &&
                            rcu_segcblist_first_pend_cb(&rdp->cblist) != head)
                                rcu_force_quiescent_state();