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

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Date:   Sat, 11 Jun 2022 05:29:18 +0000
From:   "Zhang, Qiang1" <qiang1.zhang@...el.com>
To:     Frederic Weisbecker <frederic@...nel.org>
CC:     "paulmck@...nel.org" <paulmck@...nel.org>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "linux-kernel@...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>
> ---
>  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));

>I suspect you have based your patch on upstream tree which doesn't seem to have this one yet:
>
>   "rcu/nocb: Add/del rdp to iterate from rcuog itself"
>
>With this patch you can't wait on changes to my_rdp->nocb_toggling_rdp because
>nocb_gp_wait() now performs the list_add/list_del itself.
>
>Please rebase your patch on top of latest rcu:dev from Paul's tree. Then all you need to do is to change the wait side, something like this (untested):

Hi Frederic  thanks for your advice.
This is similar to what I said in my previous email, this change is better, I will test it.

Thanks
Zqiang


>
>diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h index fa8e4f82e60c..f36d6be4f372 100644
>--- a/kernel/rcu/tree_nocb.h
>+++ b/kernel/rcu/tree_nocb.h
>@@ -584,6 +584,15 @@ static int nocb_gp_toggle_rdp(struct rcu_data *rdp,
> 	return ret;
> }
> 
>+static void nocb_gp_sleep(struct rdp *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 +710,19 @@ 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 */
>+			rdp_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"));
>+		rdp_gp_sleep(my_rdp, cpu);
> 	} else {
> 		rnp = my_rdp->mynode;
>
> 		trace_rcu_this_gp(rnp, my_rdp, wait_gp_seq, TPS("StartWait"));