linux-kernel - Re: [PATCH tip/core/rcu 06/24] rcu: Cleanup rcu_init

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Date:	Thu, 14 May 2015 22:15:29 +0100
From:	Alexander Gordeev <agordeev@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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, peterz@...radead.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH tip/core/rcu 06/24] rcu: Cleanup rcu_init_geometry() code
 and arithmetics

On Tue, May 12, 2015 at 11:22:32PM -0400, Steven Rostedt wrote:
> On Tue, 12 May 2015 15:30:36 -0700
> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > @@ -4103,24 +4102,21 @@ static void __init rcu_init_geometry(void)
> >  		return;
> >  	}
> >  
> > +	/* Calculate the number of levels in the tree. */
> > +	for (i = 0; nr_cpu_ids > rcu_capacity[i]; i++) {
> 
> Should this start at i = 1 as it use to? Also, should there be a safety
> check too:
> 
> 	for (i = 1; i <= MAX_RCU_LVLS && nr_cpu_ids > rcu_capacity[i]; i++) {

The safety check is not needed as it indirectly tried few lines above:

	if (nr_cpu_ids > rcu_capacity[MAX_RCU_LVLS])
		panic("rcu_init_geometry: rcu_capacity[] is too small");


Starting at i = 0 appears indeed incorrect. In case NR_CPUS of 1 that might
yield rcu_num_lvls = 0, which is wrong. I will check it.


> 
> > +	}
> > +	rcu_num_lvls = i;
> > +
> >  	/* Calculate the number of rcu_nodes at each level of the tree. */
> > -	for (i = 1; i <= MAX_RCU_LVLS; i++)
> > -		if (nr_cpu_ids <= rcu_capacity[i]) {
> > -			for (j = 0; j <= i; j++) {
> > -				int cap = rcu_capacity[i - j];
> > -				num_rcu_lvl[j] = DIV_ROUND_UP(nr_cpu_ids, cap);

1. In case j == i, num_rcu_lvl[j] = nr_cpu_ids

> > -			}
> > -			rcu_num_lvls = i;
> > -			for (j = i + 1; j <= MAX_RCU_LVLS; j++)
> > -				num_rcu_lvl[j] = 0;
> > -			break;
> > -		}
> > +	for (i = 0; i < rcu_num_lvls; i++) {
> 
> Hmm, up above we have: for (j = 0; j <= i; j++)
> 
> and now we have rcu_num_lvls = i, so shouldn't this be;
> 
>   for (i = 0; i <= rcu_num_lvls; i++)
> 
> ?

No, it should not. See [1] above and [2] below.

> -- Steve
> 
> > +		int cap = rcu_capacity[rcu_num_lvls - i];
> > +		num_rcu_lvl[i] = DIV_ROUND_UP(nr_cpu_ids, cap);
> > +	}
> >  
> >  	/* Calculate the total number of rcu_node structures. */
> >  	rcu_num_nodes = 0;
> > -	for (i = 0; i <= MAX_RCU_LVLS; i++)
> > +	for (i = 0; i < rcu_num_lvls; i++)
> >  		rcu_num_nodes += num_rcu_lvl[i];
> > -	rcu_num_nodes -= nr_cpu_ids;

So nr_cpu_ids is added rcu_num_nodes in the cycle and subtracted from
rcu_num_nodes afterwards.

The new version of code does not neigher [1] nor [2].

> >  }
> >  
> >  void __init rcu_init(void)
> 

-- 
Regards,
Alexander Gordeev
agordeev@...hat.com
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