netdev - RE: [PATCH 06/19] netfilter: nf_conntrack: use atomic64 for accounting counters

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Date:	Tue, 03 Jan 2012 14:31:30 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	pablo@...filter.org, netfilter-devel@...r.kernel.org,
	davem@...emloft.net, netdev@...r.kernel.org
Subject: RE: [PATCH 06/19] netfilter: nf_conntrack: use atomic64 for
 accounting counters

Le mardi 03 janvier 2012 à 12:01 +0000, David Laight a écrit :
> >  		if (acct) {
> > -			spin_lock_bh(&ct->lock);
> > -			acct[CTINFO2DIR(ctinfo)].packets++;
> > -			acct[CTINFO2DIR(ctinfo)].bytes += skb->len;
> > -			spin_unlock_bh(&ct->lock);
> > +			atomic64_inc(&acct[CTINFO2DIR(ctinfo)].packets);
> > +			atomic64_add(skb->len,
> &acct[CTINFO2DIR(ctinfo)].bytes);
> >  		}
> 
> On a 32bit arch the two atomic64 operations require a locked
> bus cycle each. The spin_unlock_bh() may not need one - so
> the code may now be slower (modulo lock contention etc).
> 
> Probably worth caching &acct[CTINFO2DIR(ctinfo)] in a local,
> the compiler probably can't do it itself.

You're mistaken.

Compile a UP kernel and check yourself before doing such claims.



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