Subject: Re: [PATCH 2/4] macvlan: cleanup rx statistics

From: Arnd Bergmann <[email-masked]>
Date: Tue, 24 Nov 2009 08:45:14 +0000

On Tuesday 24 November 2009 08:15:53 Eric Dumazet wrote:
> Arnd Bergmann a écrit :
> >  
> > +static inline void macvlan_count_rx(const struct macvlan_dev *vlan, int length,
> > +			     int success, int multicast)
> 
> success and multicast should be declared as bool

ok

> > +{
> > +	struct macvlan_rx_stats *rx_stats;
> > +
> > +	rx_stats = per_cpu_ptr(vlan->rx_stats, smp_processor_id());
> > +	rx_stats->rx_packets += success != 0;
> > +	rx_stats->rx_bytes   += success ? length : 0;
> > +	rx_stats->multicast  += success && multicast;
> > +	rx_stats->rx_errors  += !success;
> > +}
> > +
> 
> I find following more readable, it probably generates more branches,
> but avoid dirtying rx_errors if it is in another cache line.
> 
> if (likely(success)) {
> 	rx_stats->rx_packets++;
> 	rx_stats->rx_bytes += length;
> 	if (multicast)
> 		rx_stats->multicast++;
> } else {
> 	rx_stats->rx_errors++;
> }

Given that the structure only has four members and alloc_percpu requests
cache aligned data, it is rather likely to be in the same cache line.

I'll have a look at what gcc generates on x86-64 for both versions
and use the version you suggested unless it looks significantly more
expensive.

Since we're into micro-optimization territory, do you think it should
be marked inline or not?
 
> > -	rx_stats = per_cpu_ptr(vlan->rx_stats, smp_processor_id());
> >  	skb = skb_share_check(skb, GFP_ATOMIC);
> > -	if (skb == NULL) {
> > -		rx_stats->rx_errors++;
> > -		return NULL;
> > -	}
> > -
> > -	rx_stats->rx_bytes += skb->len + ETH_HLEN;
> > -	rx_stats->rx_packets++;
> > +	macvlan_count_rx(vlan, skb->len + ETH_HLEN, likely(skb != NULL), 0);
> 
> its not _likely_ that skb != NULL, its a fact :)
> 
> -> macvlan_count_rx(vlan, skb->len + ETH_HLEN, true, false);

I don't understand. Note how I removed the check for NULL above and
the skb pointer may be the result of a failing skb_clone().

Looking at this again, I actually introduced a bug by calling netif_rx
on a possibly NULL skb, I'll fix that.

Thanks!

	Arnd <><