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

Neeraj Upadhyay
2022-06-21T05:35:24Z

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);...
}

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);