linux-kernel - Re: [PATCH v2] rcu/nocb: Avoid polling when myrdp->nocb_head

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Date:   Tue, 7 Jun 2022 09:16:20 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Zqiang <qiang1.zhang@...el.com>
Cc:     frederic@...nel.org, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] rcu/nocb: Avoid polling when myrdp->nocb_head_rdp
 list is empty

On Tue, Jun 07, 2022 at 03:50:57PM +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@...el.com>

This looks a bit more plausible, but what have you done to test this?

							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.
> 
>  kernel/rcu/tree_nocb.h | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index fa8e4f82e60c..2a52c9abc681 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)) {
> +			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"));
> @@ -1030,7 +1034,7 @@ static long rcu_nocb_rdp_deoffload(void *arg)
>  
>  	mutex_lock(&rdp_gp->nocb_gp_kthread_mutex);
>  	if (rdp_gp->nocb_gp_kthread) {
> -		if (wake_gp)
> +		if (wake_gp || rcu_nocb_poll)
>  			wake_up_process(rdp_gp->nocb_gp_kthread);
>  
>  		/*
> @@ -1152,7 +1156,7 @@ static long rcu_nocb_rdp_offload(void *arg)
>  	 *      rcu_nocb_unlock()         rcu_nocb_unlock()
>  	 */
>  	wake_gp = rdp_offload_toggle(rdp, true, flags);
> -	if (wake_gp)
> +	if (wake_gp || rcu_nocb_poll)
>  		wake_up_process(rdp_gp->nocb_gp_kthread);
>  	swait_event_exclusive(rdp->nocb_state_wq,
>  			      rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB) &&
> -- 
> 2.25.1
>