linux-kernel - Re: [PATCH 2/3] rcu: Remove fast path from __rcu_process

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Message-ID: <20150211161105.GG4166@linux.vnet.ibm.com>
Date:	Wed, 11 Feb 2015 08:11:05 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Alexander Gordeev <agordeev@...hat.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] rcu: Remove fast path from __rcu_process_callbacks()

On Wed, Feb 11, 2015 at 03:42:38PM +0100, Alexander Gordeev wrote:
> The standard code path accommodates a condition when no
> RCU callbacks are ready to invoke. Since size of the code
> is a priority for tiny RCU, remove the fast path.
> 
> Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> Signed-off-by: Alexander Gordeev <agordeev@...hat.com>

Also a good point.  The savings is small in production builds because
the RCU_TRACE() statements are not compiled, but every little bit helps,
and the simplification is good as well.

Queued for 3.21.

							Thanx, Paul

> ---
>  kernel/rcu/tiny.c | 11 -----------
>  1 file changed, 11 deletions(-)
> 
> diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
> index d4e7fe5..069742d 100644
> --- a/kernel/rcu/tiny.c
> +++ b/kernel/rcu/tiny.c
> @@ -168,17 +168,6 @@ static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp)
>  	unsigned long flags;
>  	RCU_TRACE(int cb_count = 0);
> 
> -	/* If no RCU callbacks ready to invoke, just return. */
> -	if (&rcp->rcucblist == rcp->donetail) {
> -		RCU_TRACE(trace_rcu_batch_start(rcp->name, 0, 0, -1));
> -		RCU_TRACE(trace_rcu_batch_end(rcp->name, 0,
> -					      !!ACCESS_ONCE(rcp->rcucblist),
> -					      need_resched(),
> -					      is_idle_task(current),
> -					      false));
> -		return;
> -	}
> -
>  	/* Move the ready-to-invoke callbacks to a local list. */
>  	local_irq_save(flags);
>  	RCU_TRACE(trace_rcu_batch_start(rcp->name, 0, rcp->qlen, -1));
> -- 
> 1.8.3.1
> 

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