On Sat, Jun 11, 2022 at 07:00:44PM +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.
>
> Co-developed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
Much better, thank you! One additional question below.
(And we of course need Frederic's "D'accord" before I send this sort of
thing to mainline.)
Thanx, Paul
> ---
> v1->v2:
> Move rcu_nocb_poll flags check from rdp_offload_toggle() to
> rcu_nocb_rdp_offload/deoffload(), avoid unnecessary setting of
> rdp_gp->nocb_gp_sleep flags, because when rcu_nocb_poll is set the
> rdp_gp->nocb_gp_sleep is not used.
>
> v2->v3:
> When nocb_head_rdp list is empty. put rcuog kthreads in nocb_gp_wq
> waitqueue to wait offloading.
>
> kernel/rcu/tree_nocb.h | 24 +++++++++++++++++++-----
> 1 file changed, 19 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index fa8e4f82e60c..a8f574d8850d 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -584,6 +584,14 @@ static int nocb_gp_toggle_rdp(struct rcu_data *rdp,
> return ret;
> }
>
> +static void nocb_gp_sleep(struct rcu_data *my_rdp, int cpu)
> +{
> + trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("Sleep"));
> + swait_event_interruptible_exclusive(my_rdp->nocb_gp_wq,
> + !READ_ONCE(my_rdp->nocb_gp_sleep));
> + trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("EndSleep"));
> +}
> +
> /*
> * No-CBs GP kthreads come here to wait for additional callbacks to show up
> * or for grace periods to end.
> @@ -701,13 +709,19 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
> /* 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)) {
> + raw_spin_lock_irqsave(&my_rdp->nocb_gp_lock, flags);
> + if (!my_rdp->nocb_toggling_rdp)
If this "if" condition is false, what prevents this kthread from being
in a CPU-bound loop?
> + WRITE_ONCE(my_rdp->nocb_gp_sleep, true);
> + raw_spin_unlock_irqrestore(&my_rdp->nocb_gp_lock, flags);
> + /* Wait for any offloading rdp */
> + nocb_gp_sleep(my_rdp, cpu);
> + } else {
> + schedule_timeout_idle(1);
> + }
> } else if (!needwait_gp) {
> /* Wait for callbacks to appear. */
> - trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("Sleep"));
> - swait_event_interruptible_exclusive(my_rdp->nocb_gp_wq,
> - !READ_ONCE(my_rdp->nocb_gp_sleep));
> - trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("EndSleep"));
> + nocb_gp_sleep(my_rdp, cpu);
> } else {
> rnp = my_rdp->mynode;
> trace_rcu_this_gp(rnp, my_rdp, wait_gp_seq, TPS("StartWait"));
> --
> 2.25.1
>
