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Message-ID: <87seyjwgme.fsf@cloudflare.com>
Date: Wed, 15 May 2024 11:48:09 +0200
From: Jakub Sitnicki <jakub@...udflare.com>
To: Feng zhou <zhoufeng.zf@...edance.com>
Cc: edumazet@...gle.com, ast@...nel.org, daniel@...earbox.net,
andrii@...nel.org, martin.lau@...ux.dev, eddyz87@...il.com,
song@...nel.org, yonghong.song@...ux.dev, john.fastabend@...il.com,
kpsingh@...nel.org, sdf@...gle.com, haoluo@...gle.com,
jolsa@...nel.org, davem@...emloft.net, dsahern@...nel.org,
kuba@...nel.org, pabeni@...hat.com, laoar.shao@...il.com,
netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
bpf@...r.kernel.org, yangzhenze@...edance.com,
wangdongdong.6@...edance.com
Subject: Re: [PATCH bpf-next] bpf: tcp: Improve bpf write tcp opt performance
On Wed, May 15, 2024 at 04:19 PM +08, Feng zhou wrote:
> From: Feng Zhou <zhoufeng.zf@...edance.com>
>
> Set the full package write tcp option, the test found that the loss
> will be 20%. If a package wants to write tcp option, it will trigger
> bpf prog three times, and call "tcp_send_mss" calculate mss_cache,
> call "tcp_established_options" to reserve tcp opt len, call
> "bpf_skops_write_hdr_opt" to write tcp opt, but "tcp_send_mss" before
> TSO. Through bpftrace tracking, it was found that during the pressure
> test, "tcp_send_mss" call frequency was 90w/s. Considering that opt
> len does not change often, consider caching opt len for optimization.
You could also make your BPF sock_ops program cache the value and return
the cached value when called for BPF_SOCK_OPS_HDR_OPT_LEN_CB.
If that is in your opinion prohibitevely expensive then it would be good
to see a sample program and CPU cycle measurements (bpftool prog profile).
>
> Signed-off-by: Feng Zhou <zhoufeng.zf@...edance.com>
> ---
> include/linux/tcp.h | 3 +++
> include/uapi/linux/bpf.h | 8 +++++++-
> net/ipv4/tcp_output.c | 12 +++++++++++-
> tools/include/uapi/linux/bpf.h | 8 +++++++-
> 4 files changed, 28 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/tcp.h b/include/linux/tcp.h
> index 6a5e08b937b3..74437fcf94a2 100644
> --- a/include/linux/tcp.h
> +++ b/include/linux/tcp.h
> @@ -455,6 +455,9 @@ struct tcp_sock {
> * to recur itself by calling
> * bpf_setsockopt(TCP_CONGESTION, "itself").
> */
> + u8 bpf_opt_len; /* save tcp opt len implementation
> + * BPF_SOCK_OPS_HDR_OPT_LEN_CB fast path
> + */
> #define BPF_SOCK_OPS_TEST_FLAG(TP, ARG) (TP->bpf_sock_ops_cb_flags & ARG)
> #else
> #define BPF_SOCK_OPS_TEST_FLAG(TP, ARG) 0
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 90706a47f6ff..f2092de1f432 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -6892,8 +6892,14 @@ enum {
> * options first before the BPF program does.
> */
> BPF_SOCK_OPS_WRITE_HDR_OPT_CB_FLAG = (1<<6),
> + /* Fast path to reserve space in a skb under
> + * sock_ops->op == BPF_SOCK_OPS_HDR_OPT_LEN_CB.
> + * opt length doesn't change often, so it can save in the tcp_sock. And
> + * set BPF_SOCK_OPS_HDR_OPT_LEN_CACHE_CB_FLAG to no bpf call.
> + */
> + BPF_SOCK_OPS_HDR_OPT_LEN_CACHE_CB_FLAG = (1<<7),
> /* Mask of all currently supported cb flags */
> - BPF_SOCK_OPS_ALL_CB_FLAGS = 0x7F,
> + BPF_SOCK_OPS_ALL_CB_FLAGS = 0xFF,
> };
>
> /* List of known BPF sock_ops operators.
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index ea7ad7d99245..0e7480a58012 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -488,12 +488,21 @@ static void bpf_skops_hdr_opt_len(struct sock *sk, struct sk_buff *skb,
> {
> struct bpf_sock_ops_kern sock_ops;
> int err;
> + struct tcp_sock *th = (struct tcp_sock *)sk;
>
> - if (likely(!BPF_SOCK_OPS_TEST_FLAG(tcp_sk(sk),
> + if (likely(!BPF_SOCK_OPS_TEST_FLAG(th,
> BPF_SOCK_OPS_WRITE_HDR_OPT_CB_FLAG)) ||
> !*remaining)
> return;
>
> + if (likely(BPF_SOCK_OPS_TEST_FLAG(th,
> + BPF_SOCK_OPS_HDR_OPT_LEN_CACHE_CB_FLAG)) &&
> + th->bpf_opt_len) {
> + *remaining -= th->bpf_opt_len;
What if *remaining value shrinks from one call to the next?
BPF sock_ops program can't react to change. Feels like there should be a
safety check to prevent an underflow.
> + opts->bpf_opt_len = th->bpf_opt_len;
> + return;
> + }
> +
> /* *remaining has already been aligned to 4 bytes, so *remaining >= 4 */
>
> /* init sock_ops */
> @@ -538,6 +547,7 @@ static void bpf_skops_hdr_opt_len(struct sock *sk, struct sk_buff *skb,
> opts->bpf_opt_len = *remaining - sock_ops.remaining_opt_len;
> /* round up to 4 bytes */
> opts->bpf_opt_len = (opts->bpf_opt_len + 3) & ~3;
> + th->bpf_opt_len = opts->bpf_opt_len;
>
> *remaining -= opts->bpf_opt_len;
> }
> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index 90706a47f6ff..f2092de1f432 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/include/uapi/linux/bpf.h
> @@ -6892,8 +6892,14 @@ enum {
> * options first before the BPF program does.
> */
> BPF_SOCK_OPS_WRITE_HDR_OPT_CB_FLAG = (1<<6),
> + /* Fast path to reserve space in a skb under
> + * sock_ops->op == BPF_SOCK_OPS_HDR_OPT_LEN_CB.
> + * opt length doesn't change often, so it can save in the tcp_sock. And
> + * set BPF_SOCK_OPS_HDR_OPT_LEN_CACHE_CB_FLAG to no bpf call.
> + */
> + BPF_SOCK_OPS_HDR_OPT_LEN_CACHE_CB_FLAG = (1<<7),
Have you considered a bpf_reserve_hdr_opt() flag instead?
An example or test coverage would to show this API extension in action
would help.
> /* Mask of all currently supported cb flags */
> - BPF_SOCK_OPS_ALL_CB_FLAGS = 0x7F,
> + BPF_SOCK_OPS_ALL_CB_FLAGS = 0xFF,
> };
>
> /* List of known BPF sock_ops operators.
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