Subject: Re: [PATCH 2/2] net: neighbor timer power saving

From: Stephen Hemminger <[email-masked]>
Date: Thu, 20 Dec 2007 09:10:05 -0800

On Wed, 19 Dec 2007 08:23:43 +0100
Eric Dumazet <[email-masked]> wrote:

> Stephen Hemminger a écrit :
> > The neighbor GC timer runs once a second, but it doesn't need to wake
> > up the machine.
> > 
> > Signed-off-by: Stephen Hemminger <[email-masked]>
> > 
> > --- a/net/core/neighbour.c	2007-12-18 07:46:07.000000000 -0800
> > +++ b/net/core/neighbour.c	2007-12-18 07:47:36.000000000 -0800
> > @@ -270,7 +270,7 @@ static struct neighbour *neigh_alloc(str
> >  	n->nud_state	  = NUD_NONE;
> >  	n->output	  = neigh_blackhole;
> >  	n->parms	  = neigh_parms_clone(&tbl->parms);
> > -	init_timer(&n->timer);
> > +	init_timer_deferrable(&n->timer);
> >  	n->timer.function = neigh_timer_handler;
> >  	n->timer.data	  = (unsigned long)n;
> >  
> > @@ -740,7 +740,7 @@ static void neigh_timer_handler(unsigned
> >  
> >  	state = neigh->nud_state;
> >  	now = jiffies;
> > -	next = now + HZ;
> > +	next = round_jiffies(now + HZ);
> >  
> >  	if (!(state & NUD_IN_TIMER)) {
> >  #ifndef CONFIG_SMP
> > @@ -1372,7 +1372,7 @@ void neigh_table_init_no_netlink(struct 
> >  	get_random_bytes(&tbl->hash_rnd, sizeof(tbl->hash_rnd));
> >  
> >  	rwlock_init(&tbl->lock);
> > -	init_timer(&tbl->gc_timer);
> > +	init_timer_deferrable(&tbl->gc_timer);
> >  	tbl->gc_timer.data     = (unsigned long)tbl;
> >  	tbl->gc_timer.function = neigh_periodic_timer;
> >  	tbl->gc_timer.expires  = now + 1;
> 
> I wonder if this deferrable timer thing is the right way to go.
> 
> (like read_mostly thing if you want :) )
> 
> We are going to convert 99% timers to deferrable.
> 
> Maybe the right move should be to have the reverse attribute, to mark a timer 
> as non deferrable...
> 
> Also, why use round_jiffies() on a deferrable timer ? That sounds unecessary ?

Thinking about it more, this looks like a case for just using round_jiffies().
The GC timer needs to run to clean up under DoS attack, and deferring it probably
isn't a good idea.

-- 
Stephen Hemminger <[email-masked]>