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

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Date:   Wed, 07 Dec 2016 09:32:04 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     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, 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.