Re: [RCU] adds a prefetch() in rcu_do_batch()

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Nov 22, 2006 at 04:02:29PM +0100, Eric Dumazet wrote:
> On some workloads, (for example when lot of close() syscalls are done), RCU
> qlen can be quite large, and RCU heads are no longer in cpu cache when
> rcu_do_batch() is called.
> 
> This patches adds a prefetch() in rcu_do_batch() to give CPU a hint to bring
> back cache lines containing 'struct rcu_head's.
> 
> Most list manipulations macros include prefetch(), but not open coded ones (at
> least with current C compilers :) )
> 
> I got a nice speedup on a trivial benchmark  (3.48 us per iteration instead of
> 3.95 us on a 1.6 GHz Pentium-M)
> while (1) { pipe(p); close(fd[0]); close(fd[1]);}

Interesting!  How much of the speedup was due to the prefetch() and how
much to removing the extra store to rdp->donelist?

							Thanx, Paul

> Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>

> --- linux-2.6.19-rc6/kernel/rcupdate.c	2006-11-16 05:03:40.000000000 +0100
> +++ linux-2.6.19-rc6-ed/kernel/rcupdate.c	2006-11-22 15:12:09.000000000 +0100
> @@ -235,12 +235,14 @@ static void rcu_do_batch(struct rcu_data
> 
>  	list = rdp->donelist;
>  	while (list) {
> -		next = rdp->donelist = list->next;
> +		next = list->next;
> +		prefetch(next);
>  		list->func(list);
>  		list = next;
>  		if (++count >= rdp->blimit)
>  			break;
>  	}
> +	rdp->donelist = list;
> 
>  	local_irq_disable();
>  	rdp->qlen -= count;