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SubjectRe: [PATCH rcu 7/7] rcu/nocb: Avoid polling when my_rdp->nocb_head_rdp list is empty
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On 6/21/2022 4:15 AM, Paul E. McKenney wrote:
> From: Zqiang <qiang1.zhang@intel.com>
>
> Currently, if the 'rcu_nocb_poll' kernel boot parameter is enabled, all
> rcuog kthreads enter polling mode. However, if all of a given group
> of rcuo kthreads correspond to CPUs that have been de-offloaded, the
> corresponding rcuog kthread will nonetheless still wake up periodically,
> unnecessarily consuming power and perturbing workloads. Fortunately,
> this situation is easily detected by the fact that the rcuog kthread's
> CPU's rcu_data structure's ->nocb_head_rdp list is empty.
>
> This commit saves power and avoids unnecessarily perturbing workloads
> by putting an rcuog kthread to sleep during any time period when all of
> its rcuo kthreads' CPUs are de-offloaded.
>
> Co-developed-by: Frederic Weisbecker <frederic@kernel.org>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> ---


Reviewed-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>


Thanks
Neeraj

> 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 fa8e4f82e60c0..a8f574d8850d2 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)
> + 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"));

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