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SubjectRe: [PATCH] rcu/nocb: Avoid polling when myrdp->nocb_head_rdp list is empty
On Mon, Jun 06, 2022 at 02:51:43PM +0800, Zqiang wrote:
> Currently, If the 'rcu_nocb_poll' bootargs is enable, all rcuog kthreads
> enter polling mode. however, due to only insert CPU's rdp which belong to
> rcu_nocb_mask to 'nocb_head_rdp' list or all CPU's rdp served by rcuog
> kthread have been de-offloaded, these cause the 'nocb_head_rdp' list
> served by rcuog kthread is empty, when the 'nocb_head_rdp' is empty,
> the rcuog kthread in polling mode not actually do anything. fix it by
> exiting polling mode when the 'nocb_head_rdp'list is empty, otherwise
> entering polling mode.
>
> Signed-off-by: Zqiang <qiang1.zhang@intel.com>

Looks promising! A couple of questions below.

> ---
> kernel/rcu/tree_nocb.h | 14 +++++++++-----
> 1 file changed, 9 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index fa8e4f82e60c..c25c3e3dd22d 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -698,10 +698,14 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
> TPS("WakeBypassIsDeferred"));
> }
> if (rcu_nocb_poll) {
> - /* Polling, so trace if first poll in the series. */
> - if (gotcbs)
> - trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("Poll"));
> - schedule_timeout_idle(1);
> + if (list_empty(&my_rdp->nocb_head_rdp)) {

Do we need to set rdp_gp->nocb_gp_sleep to true in here somewhere?

> + rcu_wait(READ_ONCE(my_rdp->nocb_toggling_rdp));
> + } else {
> + /* Polling, so trace if first poll in the series. */
> + if (gotcbs)
> + trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("Poll"));
> + schedule_timeout_idle(1);
> + }
> } else if (!needwait_gp) {
> /* Wait for callbacks to appear. */
> trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("Sleep"));
> @@ -980,7 +984,7 @@ static int rdp_offload_toggle(struct rcu_data *rdp,
> raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);
> // Queue this rdp for add/del to/from the list to iterate on rcuog
> WRITE_ONCE(rdp_gp->nocb_toggling_rdp, rdp);
> - if (rdp_gp->nocb_gp_sleep) {
> + if (rcu_nocb_poll || rdp_gp->nocb_gp_sleep) {

Is it the case that other than when (de-)offloading, the rcuog kthread
is never awakened when rcu_nocb_poll is set? If this is not the case,
we would have spurious wakeups. (I believe that rcu_nocb_poll suppresses
those wakeups, but it would be good to check.)

> rdp_gp->nocb_gp_sleep = false;
> wake_gp = true;
> }
> --
> 2.25.1
>

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