On Sat, 8 Dec 2018 10:57:58 +0100
Florian Westphal <f...@strlen.de> wrote:
> Jesper Dangaard Brouer <bro...@redhat.com> wrote:
> > From: Ilias Apalodimas <ilias.apalodi...@linaro.org>
> >
> > This patch is changing struct sk_buff, and is thus per-definition
> > controversial.
> >
> > Place a new member 'mem_info' of type struct xdp_mem_info, just after
> > members (flags) head_frag and pfmemalloc, And not in between
> > headers_start/end to ensure skb_copy() and pskb_copy() work as-is.
> > Copying mem_info during skb_clone() is required. This makes sure that
> > pages are correctly freed or recycled during the altered
> > skb_free_head() invocation.
>
> I read this to mean that this 'info' isn't accessed/needed until skb
> is freed. Any reason its not added at the end?
>
> This would avoid moving other fields that are probably accessed
> more frequently during processing.
It is placed here because it is close to skb->head_frag, which is used
also used when SKB is freed. This is the reason, both because I think
we can move the skb->head_frag bit into mem_info, and due to cacheline
accesses (e.g. skb->cloned and destructor pointer are also read, which
is on that cache-line)
The annoying part is actually that depending on the kernel config
options CONFIG_XFRM, CONFIG_NF_CONNTRACK and CONFIG_BRIDGE_NETFILTER,
whether there is a cache-line split, where mem_info gets moved into the
next cacheline.
I do like the idea of moving it to the end, iif we also move
skb->head_frag into mem_info, as skb->head (on end-cacheline) is also
accessed during free, but given we still need to read the cache-line
containing skb->{cloned,destructor}, when I don't think it will be a
win. I think the skb->cloned value is read during lifetime of the SKB.
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer
