linux-kernel - Re: [PATCH RFC tip/core/rcu 5/5] rcu: Limit expedited helping to every 10 ms or every 4th GP

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Message-ID: <20150701100730.GQ19282@twins.programming.kicks-ass.net>
Date:	Wed, 1 Jul 2015 12:07:30 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH RFC tip/core/rcu 5/5] rcu: Limit expedited helping to
 every 10 ms or every 4th GP

On Tue, Jun 30, 2015 at 02:48:30PM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>

This seems like a good place to explain why this is a desirable thing,
no?

Why would you want to limit this?

> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> ---
>  kernel/rcu/tree.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 308b6acb4260..247aa1120c4c 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3505,10 +3505,19 @@ void synchronize_sched_expedited(void)
>  			   !atomic_read(&rsp->expedited_need_qs));
>  
>  	rcu_exp_gp_seq_end(rsp);
> -	mutex_unlock(&rnp->exp_funnel_mutex);
>  	smp_mb(); /* ensure subsequent action seen after grace period. */
> -	if (rsp->gp_kthread && rcu_gp_in_progress(rsp))
> -		wake_up(&rsp->gp_wq);
> +	if (rsp->gp_kthread && rcu_gp_in_progress(rsp)) {
> +		static unsigned long nextgp;
> +		static unsigned long nextjiffy;
> +
> +		if (time_after_eq(jiffies, nextgp) ||
> +		    ULONG_CMP_GE(rsp->gpnum, nextgp)) {
> +			nextgp = rsp->gpnum + 4;
> +			nextjiffy = jiffies + 10;
> +			wake_up(&rsp->gp_wq);
> +		}
> +	}
> +	mutex_unlock(&rnp->exp_funnel_mutex);
>  
>  	put_online_cpus();
>  }
> -- 
> 1.8.1.5
> 
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