[PATCH rcu 01/12] rcu: Decrease FQS scan wait time in case of callback overloading

Paul E. McKenney
2022-06-20T22:20:48Z

The force-quiesce-state loop function rcu_gp_fqs_loop() checks for
callback overloading and does an immediate initial scan for idle CPUs
if so. However, subsequent rescans will be carried out at as leisurely a
rate as they always are, as specified by the rcutree.jiffies_till_next_fqs
module parameter. It might be tempting to just continue immediately
rescanning, but this turns the RCU grace-period kthread into a CPU hog.
It might also be tempting to reduce the time between rescans to a single
jiffy, but this can be problematic on larger systems.

This commit therefore divides the normal time between rescans by three,
rounding up. Thus a small system running at HZ=1000 that is suffering
from callback overload will wait only one jiffy instead of the normal
three between rescans.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/rcu/tree.c | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index c25ba442044a6..c19d5926886fb 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1993,6 +1993,11 @@ static noinline_for_stack void rcu_gp_fqs_loop(void)
WRITE_ONCE(rcu_state.jiffies_kick_kthreads,
jiffies + (j ? 3 * j : 2));
}
+ if (rcu_state.cbovld) {
+ j = (j + 2) / 3;
+ if (j <= 0)
+ j = 1;
+ }
trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq,
TPS("fqswait"));
WRITE_ONCE(rcu_state.gp_state, RCU_GP_WAIT_FQS);
--
2.31.1.189.g2e36527f23