Re: [RFC PATCH net-next v1 1/2] net: add SO_NO_RFS to exclude a socket from RFS and aRFS

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Vitaly Zuevsky wrote:
> RFS and accelerated RFS are enabled per host and per rx queue, but
> whether steering helps is a property of the flow. A host that carries
> a few long-lived flows which benefit from steering alongside a large
> churn of short-lived flows has no way to express that today.
> 
> Add a boolean socket option, SO_NO_RFS. When set, sock_rps_record_flow()
> does not record the socket in rps_sock_flow_table, so get_rps_cpu()
> finds no match and neither software RFS nor ndo_rx_flow_steer() acts
> on the flow; it stays on the CPU chosen by RSS or RPS.
> 
> The option is kept in sk_flags so that sk_clone_lock() copies it to
> accepted children: setting it on a listening socket excludes every
> connection accepted from it, while sockets the same process connects
> itself remain eligible.
> 
> sock_rps_delete_flow() is skipped as well, because a socket that never
> recorded itself must not clear a table slot that may belong to another
> flow.
> 
> The test sits after the TCP_ESTABLISHED check and inside the
> rfs_needed static branch, so hosts without RFS pay nothing and hosts
> with RFS pay one test_bit() on a cache line tcp_recvmsg() already
> touches.
> 
> Signed-off-by: Vitaly Zuevsky <vitaly.zuevsky@gcore.com>

The Documentation change can/should be included in the main patch.

> ---
>  arch/alpha/include/uapi/asm/socket.h  |  2 ++
>  arch/mips/include/uapi/asm/socket.h   |  2 ++
>  arch/parisc/include/uapi/asm/socket.h |  2 ++
>  arch/sparc/include/uapi/asm/socket.h  |  2 ++
>  include/net/rps.h                     | 13 +++++++++++++
>  include/net/sock.h                    |  1 +
>  include/uapi/asm-generic/socket.h     |  2 ++
>  net/core/sock.c                       |  8 ++++++++
>  8 files changed, 32 insertions(+)
> 

> diff --git a/include/net/rps.h b/include/net/rps.h
> index e33c6a2fa8bb..5afa48868236 100644
> --- a/include/net/rps.h
> +++ b/include/net/rps.h
> @@ -98,6 +98,13 @@ static inline void _sock_rps_record_flow(const struct sock *sk)
>  	 * [1] : sk_state and sk_prot are in the same cache line.
>  	 */
>  	if (sk->sk_state == TCP_ESTABLISHED) {
> +		/* SO_NO_RFS: application asked us to never steer this flow.
> +		 * Tested after sk_state so the common case stays on one
> +		 * cache line.
> +		 */

Avoid verbose comments. Bots comment a lot more than kernel style.

Also arguably incorrect as TCP_ESTABLISHED is the common case.

> +		if (unlikely(sock_flag(sk, SOCK_NO_RFS)))
> +			return;
> +
>  		/* This READ_ONCE() is paired with the WRITE_ONCE()
>  		 * from sock_rps_save_rxhash() and sock_rps_reset_rxhash().
>  		 */
> @@ -111,6 +118,12 @@ static inline void _sock_rps_delete_flow(const struct sock *sk)
>  	rps_tag_ptr tag_ptr;
>  	u32 hash, index;
>  
> +	/* Never recorded, so never clear a slot that may belong to
> +	 * another flow.

Never is too strong here: can be set during socket lifetime.

Even if it had been correct, still another example of unnecessary
verbosity.

> +	 */
> +	if (sock_flag(sk, SOCK_NO_RFS))
> +		return;
> +

Both here and _sock_rfs_record_flow: probably should be in their
callers, like the other precondition check rfs_is_needed.

>  	hash = READ_ONCE(sk->sk_rxhash);

Optionally maybe also skip writing sk_rxhash if this is the only user.

>  	if (!hash)
>  		return;