linux-kernel - RE: [PATCH v3] rcu/nocb: Avoid polling when myrdp->nocb_head

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Date:   Sat, 11 Jun 2022 23:30:25 +0000
From:   "Zhang, Qiang1" <qiang1.zhang@...el.com>
To:     "paulmck@...nel.org" <paulmck@...nel.org>
CC:     "frederic@...nel.org" <frederic@...nel.org>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v3] rcu/nocb: Avoid polling when myrdp->nocb_head_rdp list
 is empty

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@...nel.org>
> Signed-off-by: Zqiang <qiang1.zhang@...el.com>

>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?

If this "if" condition is false, (my_rdp->nocb_toggling_rdp is not NULL),  it means the offload action
be executed, because the 'my_rdp->nocb_toggling_rdp' will only be set 'rdp' pointer for (de-)offload. 
It also means this kthread  will insert 'rdp' into 'my_rdp->nocb_head_rdp' list. in the next cycle the
'my_rdp->nocb_head_rdp' is not empty.

Thanks
Zqiang

> +				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
>