linux-kernel - Re: [PATCH rcu 7/7] rcu/nocb: Avoid polling when my_rdp->nocb_head

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Date:   Tue, 19 Jul 2022 15:06:02 +0530
From:   Neeraj Upadhyay <quic_neeraju@...cinc.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>, <rcu@...r.kernel.org>
CC:     <linux-kernel@...r.kernel.org>, <kernel-team@...com>,
        <rostedt@...dmis.org>, Zqiang <qiang1.zhang@...el.com>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH rcu 7/7] rcu/nocb: Avoid polling when
 my_rdp->nocb_head_rdp list is empty



On 6/21/2022 4:15 AM, Paul E. McKenney wrote:
> From: Zqiang <qiang1.zhang@...el.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@...nel.org>
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> Signed-off-by: Zqiang <qiang1.zhang@...el.com>
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> ---


Reviewed-by: Neeraj Upadhyay <quic_neeraju@...cinc.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"));