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Message-Id: <20220203160945.13009-1-alexandr.lobakin@intel.com>
Date: Thu, 3 Feb 2022 17:09:45 +0100
From: Alexander Lobakin <alexandr.lobakin@...el.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>,
Alexander Lobakin <alexandr.lobakin@...el.com>,
Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next 2/3] net: gro: minor optimization for dev_gro_receive()
From: Paolo Abeni <pabeni@...hat.com>
Date: Thu, 3 Feb 2022 16:48:22 +0100
> While inspecting some perf report, I noticed that the compiler
> emits suboptimal code for the napi CB initialization, fetching
> and storing multiple times the memory for flags bitfield.
> This is with gcc 10.3.1, but I observed the same with older compiler
> versions.
>
> We can help the compiler to do a nicer work clearing several
> fields at once using an u32 alias. The generated code is quite
> smaller, with the same number of conditional.
>
> Before:
> objdump -t net/core/gro.o | grep " F .text"
> 0000000000000bb0 l F .text 0000000000000357 dev_gro_receive
>
> After:
> 0000000000000bb0 l F .text 000000000000033c dev_gro_receive
>
> RFC -> v1:
> - use __struct_group to delimt the zeroed area (Alexander)
"delimit"?
>
> Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> ---
> include/net/gro.h | 52 +++++++++++++++++++++++++----------------------
> net/core/gro.c | 18 +++++++---------
> 2 files changed, 35 insertions(+), 35 deletions(-)
>
> diff --git a/include/net/gro.h b/include/net/gro.h
> index 8f75802d50fd..fa1bb0f0ad28 100644
> --- a/include/net/gro.h
> +++ b/include/net/gro.h
> @@ -29,46 +29,50 @@ struct napi_gro_cb {
> /* Number of segments aggregated. */
> u16 count;
>
> - /* Start offset for remote checksum offload */
> - u16 gro_remcsum_start;
> + /* Used in ipv6_gro_receive() and foo-over-udp */
> + u16 proto;
>
> /* jiffies when first packet was created/queued */
> unsigned long age;
>
> - /* Used in ipv6_gro_receive() and foo-over-udp */
> - u16 proto;
> + /* portion of the cb set to zero at every gro iteration */
> + __struct_group(/* no tag */, zeroed, /* no attrs */,
Oh, only after sending this in my reply I noticed that there's
a shorthand for this
struct_group(name, ...)
means exactly
__struct_group(/* no tag */, name, /* no attrs */, ...)
Sorry for that. I got confused by that Kees used __struct_group()
directly in one place although there was a possibility to use short
struct_group() instead.
Since there should already be a v2 (see below), probably worth
changing.
> +
> + /* Start offset for remote checksum offload */
> + u16 gro_remcsum_start;
>
> - /* This is non-zero if the packet may be of the same flow. */
> - u8 same_flow:1;
> + /* This is non-zero if the packet may be of the same flow. */
> + u8 same_flow:1;
>
> - /* Used in tunnel GRO receive */
> - u8 encap_mark:1;
> + /* Used in tunnel GRO receive */
> + u8 encap_mark:1;
>
> - /* GRO checksum is valid */
> - u8 csum_valid:1;
> + /* GRO checksum is valid */
> + u8 csum_valid:1;
>
> - /* Number of checksums via CHECKSUM_UNNECESSARY */
> - u8 csum_cnt:3;
> + /* Number of checksums via CHECKSUM_UNNECESSARY */
> + u8 csum_cnt:3;
>
> - /* Free the skb? */
> - u8 free:2;
> + /* Free the skb? */
> + u8 free:2;
> #define NAPI_GRO_FREE 1
> #define NAPI_GRO_FREE_STOLEN_HEAD 2
>
> - /* Used in foo-over-udp, set in udp[46]_gro_receive */
> - u8 is_ipv6:1;
> + /* Used in foo-over-udp, set in udp[46]_gro_receive */
> + u8 is_ipv6:1;
>
> - /* Used in GRE, set in fou/gue_gro_receive */
> - u8 is_fou:1;
> + /* Used in GRE, set in fou/gue_gro_receive */
> + u8 is_fou:1;
>
> - /* Used to determine if flush_id can be ignored */
> - u8 is_atomic:1;
> + /* Used to determine if flush_id can be ignored */
> + u8 is_atomic:1;
>
> - /* Number of gro_receive callbacks this packet already went through */
> - u8 recursion_counter:4;
> + /* Number of gro_receive callbacks this packet already went through */
> + u8 recursion_counter:4;
>
> - /* GRO is done by frag_list pointer chaining. */
> - u8 is_flist:1;
> + /* GRO is done by frag_list pointer chaining. */
> + u8 is_flist:1;
> + );
>
> /* used to support CHECKSUM_COMPLETE for tunneling protocols */
> __wsum csum;
> diff --git a/net/core/gro.c b/net/core/gro.c
> index d43d42215bdb..fc56be9408c7 100644
> --- a/net/core/gro.c
> +++ b/net/core/gro.c
> @@ -435,6 +435,9 @@ static void gro_flush_oldest(struct napi_struct *napi, struct list_head *head)
> napi_gro_complete(napi, oldest);
> }
>
> +#define zeroed_len (offsetof(struct napi_gro_cb, zeroed_end) - \
> + offsetof(struct napi_gro_cb, zeroed_start))
> +
A leftover from the RFC I guess? It's not used anywhere in the code,
so it doesn't break build.
> static enum gro_result dev_gro_receive(struct napi_struct *napi, struct sk_buff *skb)
> {
> u32 bucket = skb_get_hash_raw(skb) & (GRO_HASH_BUCKETS - 1);
> @@ -459,29 +462,22 @@ static enum gro_result dev_gro_receive(struct napi_struct *napi, struct sk_buff
>
> skb_set_network_header(skb, skb_gro_offset(skb));
> skb_reset_mac_len(skb);
> - NAPI_GRO_CB(skb)->same_flow = 0;
> + BUILD_BUG_ON(sizeof_field(struct napi_gro_cb, zeroed) != sizeof(u32));
> + BUILD_BUG_ON(!IS_ALIGNED(offsetof(struct napi_gro_cb, zeroed),
> + sizeof(u32))); /* Avoid slow unaligned acc */
> + *(u32 *)&NAPI_GRO_CB(skb)->zeroed = 0;
Looks relatively elegant and self-explanatory to me, nice.
> NAPI_GRO_CB(skb)->flush = skb_is_gso(skb) || skb_has_frag_list(skb);
> - NAPI_GRO_CB(skb)->free = 0;
> - NAPI_GRO_CB(skb)->encap_mark = 0;
> - NAPI_GRO_CB(skb)->recursion_counter = 0;
> - NAPI_GRO_CB(skb)->is_fou = 0;
> NAPI_GRO_CB(skb)->is_atomic = 1;
> - NAPI_GRO_CB(skb)->gro_remcsum_start = 0;
>
> /* Setup for GRO checksum validation */
> switch (skb->ip_summed) {
> case CHECKSUM_COMPLETE:
> NAPI_GRO_CB(skb)->csum = skb->csum;
> NAPI_GRO_CB(skb)->csum_valid = 1;
> - NAPI_GRO_CB(skb)->csum_cnt = 0;
> break;
> case CHECKSUM_UNNECESSARY:
> NAPI_GRO_CB(skb)->csum_cnt = skb->csum_level + 1;
> - NAPI_GRO_CB(skb)->csum_valid = 0;
> break;
> - default:
> - NAPI_GRO_CB(skb)->csum_cnt = 0;
> - NAPI_GRO_CB(skb)->csum_valid = 0;
> }
>
> pp = INDIRECT_CALL_INET(ptype->callbacks.gro_receive,
> --
> 2.34.1
Al
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