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

Paul E. McKenney
2022-06-21T22:19:35Z

On Tue, Jun 21, 2022 at 10:59:58AM +0530, Neeraj Upadhyay wrote:
>
>
> 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@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;
> > + }
>
> 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);...
> }

Good catch, thank you! How about the updated patch shown below?

Thanx, Paul

------------------------------------------------------------------------

commit 77de092c78f549b5c28075bfee9998a525d21f84
Author: Paul E. McKenney <paulmck@kernel.org>
Date: Tue Apr 12 15:08:14 2022 -0700

rcu: Decrease FQS scan wait time in case of callback overloading

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.

[ paulmck: Apply Neeraj Upadhyay feedback. ]

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index c25ba442044a6..52094e72866e5 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1983,7 +1983,12 @@ static noinline_for_stack void rcu_gp_fqs_loop(void)
gf = RCU_GP_FLAG_OVLD;
ret = 0;
for (;;) {
- if (!ret) {
+ if (rcu_state.cbovld) {
+ j = (j + 2) / 3;
+ if (j <= 0)
+ j = 1;
+ }
+ if (!ret || time_before(jiffies + j, rcu_state.jiffies_force_qs)) {
WRITE_ONCE(rcu_state.jiffies_force_qs, jiffies + j);
/*
* jiffies_force_qs before RCU_GP_WAIT_FQS state