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

ANBZ: torvalds#155 commit d5a9a8c upstream. 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]> Signed-off-by: Guanghui Feng <[email protected]> Reviewed-by: Xunlei Pang <[email protected]>
gatieme · Dec 21, 2021 · 07da018 · 07da018
1 parent 788e43c
commit 07da018
Showing 1 changed file with 4 additions and 3 deletions.
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
@@ -491,7 +491,8 @@ void rcu_all_qs(void)
 }
 EXPORT_SYMBOL_GPL(rcu_all_qs);
 
-#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;
@@ -2620,7 +2621,7 @@ static void rcu_do_batch(struct rcu_state *rsp, 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. */
@@ -2956,7 +2957,7 @@ static void __call_rcu_core(struct rcu_state *rsp, struct rcu_data *rdp,
 			rcu_accelerate_cbs_unlocked(rsp, rdp->mynode, rdp);
 		} else {
 			/* Give the grace period a kick. */
-			rdp->blimit = LONG_MAX;
+			rdp->blimit = DEFAULT_MAX_RCU_BLIMIT;
 			if (rsp->n_force_qs == rdp->n_force_qs_snap &&
 			    rcu_segcblist_first_pend_cb(&rdp->cblist) != head)
 				force_quiescent_state(rsp);