linux-kernel - Re: Bisected post-3.9 regression: Resume takes 5 times as much time as with v3.9

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Message-ID: <20130514173739.GH24440@8bytes.org>
Date:	Tue, 14 May 2013 19:37:39 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Bjørn Mork <bjorn@...k.no>,
	"Paul E. McKenney" <paul.mckenney@...aro.org>,
	linux-kernel@...r.kernel.org
Subject: Re: Bisected post-3.9 regression: Resume takes 5 times as much time
 as with v3.9

Hi Paul,

On Mon, May 13, 2013 at 07:00:05AM -0700, Paul E. McKenney wrote:
> rcu: Fix comparison sense in rcu_needs_cpu()
> 
> Commit c0f4dfd4f (rcu: Make RCU_FAST_NO_HZ take advantage of numbered
> callbacks) introduced a bug that can result in excessively long grace
> periods.  This bug reverse the senes of the "if" statement checking
> for lazy callbacks, so that RCU takes a lazy approach when there are
> in fact non-lazy callbacks.  This can result in excessive boot, suspend,
> and resume times.
> 
> This commit therefore fixes the sense of this "if" statement.
> 
> Reported-by: Borislav Petkov <bp@...en8.de>
> Reported-by: Bjørn Mork <bjorn@...k.no>
> Reported-by: Joerg Roedel <joro@...tes.org>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

Just tested the patch, it fixes the long boot delay for me. Thanks for
fixing this.

Tested-by: Joerg Roedel <joro@...tes.org>

> 
> diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> index 641991d..63098a5 100644
> --- a/kernel/rcutree_plugin.h
> +++ b/kernel/rcutree_plugin.h
> @@ -1692,7 +1692,7 @@ int rcu_needs_cpu(int cpu, unsigned long *dj)
>  	rdtp->last_accelerate = jiffies;
>  
>  	/* Request timer delay depending on laziness, and round. */
> -	if (rdtp->all_lazy) {
> +	if (!rdtp->all_lazy) {
>  		*dj = round_up(rcu_idle_gp_delay + jiffies,
>  			       rcu_idle_gp_delay) - jiffies;
>  	} else {
> 

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