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Message-ID: <20231223161620.GF201037@kernel.org>
Date: Sat, 23 Dec 2023 16:16:20 +0000
From: Simon Horman <horms@...nel.org>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
Willem de Bruijn <willemb@...gle.com>, netdev@...r.kernel.org,
eric.dumazet@...il.com, Coco Li <lixiaoyan@...gle.com>,
David Ahern <dsahern@...nel.org>
Subject: Re: [PATCH net-next] net-device: move gso_partial_features to
net_device_read_tx
On Thu, Dec 21, 2023 at 02:07:47PM +0000, Eric Dumazet wrote:
> dev->gso_partial_features is read from tx fast path for GSO packets.
>
> Move it to appropriate section to avoid a cache line miss.
>
> Fixes: 43a71cd66b9c ("net-device: reorganize net_device fast path variables")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Coco Li <lixiaoyan@...gle.com>
> Cc: David Ahern <dsahern@...nel.org>
Thanks Eric,
FWIIW, this change looks good to me.
Reviewed-by: Simon Horman <horms@...nel.org>
I have a follow-up question below.
...
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 75c7725e5e4fdf59da55923cd803e084956b0fa0..5d1ec780122919c31e4215358d736aef3f8a0acd 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -2114,6 +2114,7 @@ struct net_device {
> const struct net_device_ops *netdev_ops;
> const struct header_ops *header_ops;
> struct netdev_queue *_tx;
> + netdev_features_t gso_partial_features;
> unsigned int real_num_tx_queues;
> unsigned int gso_max_size;
> unsigned int gso_ipv4_max_size;
While looking at this I came to wonder if it would
be worth adding a 16bit pad a little below this hunk
so that tc_to_txq sits on it's own cacheline.
I'm unsure if the access pattern of tc_to_txq makes this worthwhile.
But if so it would be a simple tweak.
With such a change in place, on top of your patch, the diff of pahole output
on x86_64 is:
@@ -7432,10 +7432,9 @@
s16 num_tc; /* 54 2 */
unsigned int mtu; /* 56 4 */
short unsigned int needed_headroom; /* 60 2 */
- struct netdev_tc_txq tc_to_txq[16]; /* 62 64 */
-
- /* XXX 2 bytes hole, try to pack */
-
+ u16 pad1; /* 62 2 */
+ /* --- cacheline 1 boundary (64 bytes) --- */
+ struct netdev_tc_txq tc_to_txq[16]; /* 64 64 */
/* --- cacheline 2 boundary (128 bytes) --- */
struct xps_dev_maps * xps_maps[2]; /* 128 16 */
struct nf_hook_entries * nf_hooks_egress; /* 144 8 */
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