netdev - Re: [PATCH] net/udp: do not touch skb->peeked unless really needed

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Date:   Wed, 07 Dec 2016 18:37:47 +0100
From:   Hannes Frederic Sowa <hannes@...essinduktion.org>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        David Laight <David.Laight@...LAB.COM>
Cc:     Paolo Abeni <pabeni@...hat.com>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH] net/udp: do not touch skb->peeked unless really needed

On Wed, Dec 7, 2016, at 18:32, Eric Dumazet wrote:
> On Wed, 2016-12-07 at 17:09 +0000, David Laight wrote:
> > From: Paolo Abeni
> > > Sent: 06 December 2016 17:08
> > ...
> > > @@ -79,6 +82,9 @@ struct udp_sock {
> > >  	int			(*gro_complete)(struct sock *sk,
> > >  						struct sk_buff *skb,
> > >  						int nhoff);
> > > +
> > > +	/* since we are prone to drops, avoid dirtying any sk cacheline */
> > > +	atomic_t		drops ____cacheline_aligned_in_smp;
> > >  };
> > 
> > Isn't that likely to create a large hole on systems with large cache lines.
> > (Same as any other use of ____cacheline_aligned_in_smp.)
> 
> Yes, I would like to avoid that, unless we come to the conclusion it is
> absolutely needed.
> 
> I feel that we could simply use a pointer, and allocate memory on
> demand, since many sockets do not ever experience a drop.
> 
> The pointer could stay in a read mostly section.
> 
> We even could use per cpu or node counter for some heavy drop cases. 

I had the same idea while discussing that with Paolo, merely using an
*atomic_t = kmalloc(sizeof(atomic_t)) out of band of the socket.

My fear was that those could be aggregated by the slab cache into one
cache line, causing even more heating on cachelines.

Bye,
Hannes