Subject: Re: rib_trie / Fix inflate_threshold_root. Now=15 size=11 bits

From: "Paul E. McKenney" <[email-masked]>
Date: Fri, 26 Jun 2009 08:30:10 -0700

On Fri, Jun 26, 2009 at 05:10:52PM +0200, Jarek Poplawski wrote:
> On Fri, Jun 26, 2009 at 03:52:55PM +0200, Robert Olsson wrote:
> > 
> > Jarek Poplawski writes:
> > 
> >  Thanks, 
> > 
> >  Should be worth testing so we synchronize_rcu instead of doing call_rcu's
> >  
> 
> Alas take 2 (nor 1) doesn't compile, so here it is again.

So the idea is to balance memory and latency, so that large changes
(those affecting the root node) get at least one synchronize_rcu(),
while smaller changes just use call_rcu(), correct?  This means that
the amount of memory awaiting an RCU grace period is limited, but
the algorithm avoids per-node synchronize_rcu() overhead.

If I understand the goal correctly, looks good!  (Give or take my
limited understanding of fib_trie and is usage, of course.)

							Thanx, Paul

> Thanks,
> Jarek P.
> --- (take 3 - for testing)
> 
>  net/ipv4/fib_trie.c |   30 ++++++++++++++++++++++++------
>  1 files changed, 24 insertions(+), 6 deletions(-)
> 
> diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c
> index 012cf5a..1a4c4b7 100644
> --- a/net/ipv4/fib_trie.c
> +++ b/net/ipv4/fib_trie.c
> @@ -366,6 +366,17 @@ static void __tnode_vfree(struct work_struct *arg)
>  	vfree(tn);
>  }
> 
> +static void __tnode_free(struct tnode *tn)
> +{
> +	size_t size = sizeof(struct tnode) +
> +		      (sizeof(struct node *) << tn->bits);
> +
> +	if (size <= PAGE_SIZE)
> +		kfree(tn);
> +	else
> +		vfree(tn);
> +}
> +
>  static void __tnode_free_rcu(struct rcu_head *head)
>  {
>  	struct tnode *tn = container_of(head, struct tnode, rcu);
> @@ -402,7 +413,7 @@ static void tnode_free_flush(void)
>  	while ((tn = tnode_free_head)) {
>  		tnode_free_head = tn->tnode_free;
>  		tn->tnode_free = NULL;
> -		tnode_free(tn);
> +		__tnode_free(tn);
>  	}
>  }
> 
> @@ -1021,18 +1032,25 @@ static void trie_rebalance(struct trie *t, struct tnode *tn)
>  				      (struct node *)tn, wasfull);
> 
>  		tp = node_parent((struct node *) tn);
> -		tnode_free_flush();
>  		if (!tp)
>  			break;
>  		tn = tp;
>  	}
> 
> +	if (tnode_free_head) {
> +		synchronize_rcu();
> +		tnode_free_flush();
> +	}
> +
>  	/* Handle last (top) tnode */
> -	if (IS_TNODE(tn))
> +	if (IS_TNODE(tn)) {
>  		tn = (struct tnode *)resize(t, (struct tnode *)tn);
> -
> -	rcu_assign_pointer(t->trie, (struct node *)tn);
> -	tnode_free_flush();
> +		rcu_assign_pointer(t->trie, (struct node *)tn);
> +		synchronize_rcu();
> +		tnode_free_flush();
> +	} else {
> +		rcu_assign_pointer(t->trie, (struct node *)tn);
> +	}
> 
>  	return;
>  }
> --
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