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Subject[PATCH tip/core/rcu 4/9] rcu: Set a maximum limit for back-to-back callback invocation
Date
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>
---
kernel/rcu/tree.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 980ca3ca643f..f888a76673da 100644
--- 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();
--
2.17.1
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