On 6/21/2022 3:50 AM, Paul E. McKenney wrote:
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@xxxxxxxxxx>
---
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;
+ }
We update 'j' here, after setting rcu_state.jiffies_force_qs
WRITE_ONCE(rcu_state.jiffies_force_qs, jiffies + j)
So, we return from swait_event_idle_timeout_exclusive after 1/3 time
duration.
swait_event_idle_timeout_exclusive(rcu_state.gp_wq,
rcu_gp_fqs_check_wake(&gf), j);
This can result in !timer_after check to return false and we will
enter the 'else' (stray signal block) code?
This might not matter for first 2 fqs loop iterations, where
RCU_GP_FLAG_OVLD is set in 'gf', but subsequent iterations won't benefit
from this patch?
if (!time_after(rcu_state.jiffies_force_qs, jiffies) ||
(gf & (RCU_GP_FLAG_FQS | RCU_GP_FLAG_OVLD))) {
...
} else {
/* Deal with stray signal. */
}
So, do we need to move this calculation above the 'if' block which sets
rcu_state.jiffies_force_qs?
if (!ret) {
WRITE_ONCE(rcu_state.jiffies_force_qs, jiffies +
j);...
}
Thanks
Neeraj
trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq,
TPS("fqswait"));
WRITE_ONCE(rcu_state.gp_state, RCU_GP_WAIT_FQS);
