linux-kernel - Re: [PATCH tip/core/rcu 20/23] rcu: Remove callback acceleration from grace-period initialization

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Date:	Mon, 3 Sep 2012 02:42:46 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, sbw@....edu, patches@...aro.org
Subject: Re: [PATCH tip/core/rcu 20/23] rcu: Remove callback acceleration
 from grace-period initialization

On Thu, Aug 30, 2012 at 11:18:35AM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> 
> Before grace-period initialization was moved to a kthread, the CPU
> invoking this code would have at least one callback that needed
> a grace period, often a newly registered callback.  However, moving
> grace-period initialization means that the CPU with the callback
> that was requesting a grace period is not necessarily the CPU that
> is initializing the grace period, so this acceleration is less
> valuable.  Because it also adds to the complexity of reasoning about
> correctness, this commit removes it.
> 
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

Reviewed-by: Josh Triplett <josh@...htriplett.org>

>  kernel/rcutree.c |   19 -------------------
>  1 files changed, 0 insertions(+), 19 deletions(-)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index 86903df..44609c3 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -1055,25 +1055,6 @@ static int rcu_gp_init(struct rcu_state *rsp)
>  	rsp->gpnum++;
>  	trace_rcu_grace_period(rsp->name, rsp->gpnum, "start");
>  	record_gp_stall_check_time(rsp);
> -
> -	/*
> -	 * Because this CPU just now started the new grace period, we
> -	 * know that all of its callbacks will be covered by this upcoming
> -	 * grace period, even the ones that were registered arbitrarily
> -	 * recently.    Therefore, advance all RCU_NEXT_TAIL callbacks
> -	 * to RCU_NEXT_READY_TAIL.  When the CPU later recognizes the
> -	 * start of the new grace period, it will advance all callbacks
> -	 * one position, which will cause all of its current outstanding
> -	 * callbacks to be handled by the newly started grace period.
> -	 *
> -	 * Other CPUs cannot be sure exactly when the grace period started.
> -	 * Therefore, their recently registered callbacks must pass through
> -	 * an additional RCU_NEXT_READY stage, so that they will be handled
> -	 * by the next RCU grace period.
> -	 */
> -	rdp = __this_cpu_ptr(rsp->rda);
> -	rdp->nxttail[RCU_NEXT_READY_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
> -
>  	raw_spin_unlock_irqrestore(&rnp->lock, flags);
>  
>  	/* Exclude any concurrent CPU-hotplug operations. */
> -- 
> 1.7.8
> 
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