From: Eric Dumazet <edumazet@google.com>
To: Alexander Lobakin <alobakin@pm.me>
Cc: "David S. Miller" <davem@davemloft.net>,
Jakub Kicinski <kuba@kernel.org>,
Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Andrii Nakryiko <andriin@fb.com>, Wei Wang <weiwan@google.com>,
Cong Wang <cong.wang@bytedance.com>,
Taehee Yoo <ap420073@gmail.com>, netdev <netdev@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH net-next 2/4] gro: don't dereference napi->gro_hash[x] multiple times in dev_gro_receive()
Date: Fri, 12 Mar 2021 17:47:04 +0100 [thread overview]
Message-ID: <CANn89iJvmakwfLFb6QS1ettiJM-D3cJ89bFPvZ=Gk2YyGpxQuw@mail.gmail.com> (raw)
In-Reply-To: <20210312162127.239795-3-alobakin@pm.me>
On Fri, Mar 12, 2021 at 5:22 PM Alexander Lobakin <alobakin@pm.me> wrote:
>
> GRO bucket index doesn't change through the entire function.
> Store a pointer to the corresponding bucket on stack once and use
> it later instead of dereferencing again and again.
>
> Signed-off-by: Alexander Lobakin <alobakin@pm.me>
> ---
> net/core/dev.c | 9 +++++----
> 1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index adc42ba7ffd8..ee124aecb8a2 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -5957,6 +5957,7 @@ static void gro_flush_oldest(struct napi_struct *napi, struct list_head *head)
> 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);
> + struct gro_list *gro_list = &napi->gro_hash[bucket];
> struct list_head *head = &offload_base;
> struct packet_offload *ptype;
> __be16 type = skb->protocol;
> @@ -6024,7 +6025,7 @@ static enum gro_result dev_gro_receive(struct napi_struct *napi, struct sk_buff
> if (pp) {
> skb_list_del_init(pp);
> napi_gro_complete(napi, pp);
> - napi->gro_hash[bucket].count--;
> + gro_list->count--;
> }
>
> if (same_flow)
> @@ -6033,10 +6034,10 @@ static enum gro_result dev_gro_receive(struct napi_struct *napi, struct sk_buff
> if (NAPI_GRO_CB(skb)->flush)
> goto normal;
>
> - if (unlikely(napi->gro_hash[bucket].count >= MAX_GRO_SKBS)) {
> + if (unlikely(gro_list->count >= MAX_GRO_SKBS)) {
> gro_flush_oldest(napi, gro_head);
> } else {
> - napi->gro_hash[bucket].count++;
> + gro_list->count++;
> }
> NAPI_GRO_CB(skb)->count = 1;
> NAPI_GRO_CB(skb)->age = jiffies;
> @@ -6050,7 +6051,7 @@ static enum gro_result dev_gro_receive(struct napi_struct *napi, struct sk_buff
> if (grow > 0)
> gro_pull_from_frag0(skb, grow);
> ok:
> - if (napi->gro_hash[bucket].count) {
> + if (gro_list->count) {
> if (!test_bit(bucket, &napi->gro_bitmask))
> __set_bit(bucket, &napi->gro_bitmask);
> } else if (test_bit(bucket, &napi->gro_bitmask)) {
> --
> 2.30.2
>
>
This adds more register pressure, do you have precise measures to
confirm this change is a win ?
Presumably the compiler should be able to optimize the code just fine,
it can see @bucket does not change.
next prev parent reply other threads:[~2021-03-12 16:47 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-12 16:21 [PATCH net-next 0/4] gro: micro-optimize dev_gro_receive() Alexander Lobakin
2021-03-12 16:21 ` [PATCH net-next 1/4] gro: give 'hash' variable in dev_gro_receive() a less confusing name Alexander Lobakin
2021-03-12 16:21 ` [PATCH net-next 2/4] gro: don't dereference napi->gro_hash[x] multiple times in dev_gro_receive() Alexander Lobakin
2021-03-12 16:47 ` Eric Dumazet [this message]
2021-03-12 18:36 ` Alexander Lobakin
2021-03-12 16:22 ` [PATCH net-next 3/4] gro: simplify gro_list_prepare() Alexander Lobakin
2021-03-12 16:22 ` [PATCH net-next 4/4] gro: improve flow distribution across GRO buckets in dev_gro_receive() Alexander Lobakin
2021-03-12 16:33 ` Eric Dumazet
2021-03-12 18:28 ` Alexander Lobakin
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