netdev - Re: [RFC bpf-next v2 10/14] ice: Support rx timestamp metadata for xdp

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Message-ID: <Y5sIUI1jeN3c7iQA@lincoln>
Date:   Thu, 15 Dec 2022 12:54:16 +0100
From:   Larysa Zaremba <larysa.zaremba@...el.com>
To:     Stanislav Fomichev <sdf@...gle.com>
CC:     <bpf@...r.kernel.org>, <ast@...nel.org>, <daniel@...earbox.net>,
        <andrii@...nel.org>, <martin.lau@...ux.dev>, <song@...nel.org>,
        <yhs@...com>, <john.fastabend@...il.com>, <kpsingh@...nel.org>,
        <haoluo@...gle.com>, <jolsa@...nel.org>,
        David Ahern <dsahern@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        "Jesper Dangaard Brouer" <brouer@...hat.com>,
        Anatoly Burakov <anatoly.burakov@...el.com>,
        Alexander Lobakin <alexandr.lobakin@...el.com>,
        Magnus Karlsson <magnus.karlsson@...il.com>,
        Maryam Tahhan <mtahhan@...hat.com>,
        <xdp-hints@...-project.net>, <netdev@...r.kernel.org>
Subject: Re: [RFC bpf-next v2 10/14] ice: Support rx timestamp metadata for
 xdp

On Thu, Nov 03, 2022 at 08:25:28PM -0700, Stanislav Fomichev wrote:
> +			/* if (r5 == NULL) return; */
> +			BPF_JMP_IMM(BPF_JNE, BPF_REG_5, 0, S16_MAX),

S16_MAX jump crashes my system and I do not see such jumps used very often
in bpf code found in-tree, setting a fixed jump length worked for me.
Also, I think BPF_JEQ is a correct condition in this case, not BPF_JNE.

But the main reason for my reply is that I have implemented RX hash hint
for ice both as unrolled bpf code and with BPF_EMIT_CALL [0].
Both bpf_xdp_metadata_rx_hash() and bpf_xdp_metadata_rx_hash_supported() 
are implemented in those 2 ways.

RX hash is the easiest hint to read, so performance difference
should be more visible than when reading timestapm.

Counting packets in an rxdrop XDP program on a single queue
gave me the following numbers:

- unrolled:		41264360 pps
- BPF_EMIT_CALL:	40370651 pps

So, reading a single hint in an unrolled way instead of calling 2 driver
functions in a row, gives us a 2.2% performance boost.
Surely, the difference will increase, if we read more than a single hint.
Therefore, it would be great to implement at least some simple hints
functions as unrolled.

[0] https://github.com/walking-machine/linux/tree/ice-kfunc-hints-clean