linux-kernel - Re: [PATCH rcu 08/12] rcu: Cleanup RCU urgency state for offline CPU

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Date:   Tue, 21 Jun 2022 12:33:12 +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>
Subject: Re: [PATCH rcu 08/12] rcu: Cleanup RCU urgency state for offline CPU



On 6/21/2022 3:50 AM, Paul E. McKenney wrote:
> From: Zqiang <qiang1.zhang@...el.com>
> 
> When a CPU is slow to provide a quiescent state for a given grace
> period, RCU takes steps to encourage that CPU to get with the
> quiescent-state program in a more timely fashion.  These steps
> include these flags in the rcu_data structure:
> 
> 1.	->rcu_urgent_qs, which causes the scheduling-clock interrupt to
> 	request an otherwise pointless context switch from the scheduler.
> 
> 2.	->rcu_need_heavy_qs, which causes both cond_resched() and RCU's
> 	context-switch hook to do an immediate momentary quiscent state.
> 
> 3.	->rcu_need_heavy_qs, which causes the scheduler-clock tick to

nit: s/->rcu_need_heavy_qs/->rcu_forced_tick/ ?

> 	be enabled even on nohz_full CPUs with only one runnable task.
> 
> These flags are of course cleared once the corresponding CPU has passed
> through a quiescent state.  Unless that quiescent state is the CPU
> going offline, which means that when the CPU comes back online, it will
> needlessly consume additional CPU time and incur additional latency,
> which constitutes a minor but very real performance bug.
> 
> This commit therefore adds the call to rcu_disable_urgency_upon_qs()
> that clears these flags to the CPU-hotplug offlining code path.
> 
> 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.c | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index f4a37f2032664..5445b19b48408 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -4446,6 +4446,7 @@ void rcu_report_dead(unsigned int cpu)
>   	rdp->rcu_ofl_gp_flags = READ_ONCE(rcu_state.gp_flags);
>   	if (rnp->qsmask & mask) { /* RCU waiting on outgoing CPU? */
>   		/* Report quiescent state -before- changing ->qsmaskinitnext! */
> +		rcu_disable_urgency_upon_qs(rdp);
>   		rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags);
>   		raw_spin_lock_irqsave_rcu_node(rnp, flags);
>   	}