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 <[email protected]> Signed-off-by: Paul E. McKenney <[email protected]>
gatieme · May 28, 2019 · d5a9a8c · d5a9a8c
1 parent 12edff0
commit d5a9a8c
Showing 1 changed file with 4 additions and 3 deletions.
diff --git 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();