linux-kernel - Re: [PATCH -mm][v2] ratelimit rewrite

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Message-ID: <20080508231952.GC13851@linux.vnet.ibm.com>
Date:	Thu, 8 May 2008 16:19:52 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Dave Young <hidave.darkstar@...il.com>, joe@...ches.com,
	davem@...emloft.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm][v2] ratelimit rewrite

On Thu, May 08, 2008 at 12:25:14PM -0700, Andrew Morton wrote:
> On Tue, 6 May 2008 10:25:58 +0800
> Dave Young <hidave.darkstar@...il.com> wrote:
> 
> >  static inline void rcu_enter_nohz(void)
> >  {
> > +	static DEFINE_RATELIMIT_STATE(rs, 10 * HZ, 1);
> >  	smp_mb(); /* CPUs seeing ++ must see prior RCU read-side crit sects */
> >  	__get_cpu_var(rcu_dyntick_sched).dynticks++;
> > -	WARN_ON_SECS(__get_cpu_var(rcu_dyntick_sched).dynticks & 0x1, 10);
> > +	WARN_ON_RATELIMIT(__get_cpu_var(rcu_dyntick_sched).dynticks & 0x1, &rs);
> >  }
> >  
> >  static inline void rcu_exit_nohz(void)
> >  {
> > +	static DEFINE_RATELIMIT_STATE(rs, 10 * HZ, 1);
> >  	__get_cpu_var(rcu_dyntick_sched).dynticks++;
> >  	smp_mb(); /* CPUs seeing ++ must see later RCU read-side crit sects */
> > -	WARN_ON_SECS(!(__get_cpu_var(rcu_dyntick_sched).dynticks & 0x1), 10);
> > +	WARN_ON_RATELIMIT(!(__get_cpu_var(rcu_dyntick_sched).dynticks & 0x1),
> > +			&rs);
> 
> Why are we altering the RCU code in this patch, btw?  It seems fairly
> random that we happened to choose these particular WARN_ONs.  Do they have
> a history of triggering?

Yep -- there was a bug in this code a few months ago that was the
motivation for Dave's WARN_ON_RATELIMIT().  The bug has since been fixed,
so I would not expect this to trigger anymore, though.

							Thanx, Paul
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