netdev - Re: [PATCH RFC bpf-next 03/15] samples: bpf: split out common bpf progs to its own file

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Date:   Sat, 29 May 2021 20:05:38 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH RFC bpf-next 03/15] samples: bpf: split out common bpf
 progs to its own file

On Fri, May 28, 2021 at 4:53 PM Kumar Kartikeya Dwivedi
<memxor@...il.com> wrote:
>
> This is done to later reuse these in a way that can be shared
> among multiple samples.
>
> We are using xdp_redirect_cpu_kern.c as a base to build further support on
> top (mostly adding a few other things missing that xdp_monitor does in
> subsequent patches).
>
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@...il.com>
> ---
>  samples/bpf/xdp_sample_kern.h | 220 ++++++++++++++++++++++++++++++++++
>  1 file changed, 220 insertions(+)
>  create mode 100644 samples/bpf/xdp_sample_kern.h
>
> diff --git a/samples/bpf/xdp_sample_kern.h b/samples/bpf/xdp_sample_kern.h

instead of doing it as a header, can you please use BPF static linking
instead? I think that's a better approach and a good showcase for
anyone that would like to use static linking for their BPF programs

> new file mode 100644
> index 000000000000..bb809542ac20
> --- /dev/null
> +++ b/samples/bpf/xdp_sample_kern.h
> @@ -0,0 +1,220 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*  GPLv2, Copyright(c) 2017 Jesper Dangaard Brouer, Red Hat, Inc. */
> +#pragma once
> +
> +#include <uapi/linux/bpf.h>
> +#include <bpf/bpf_helpers.h>
> +
> +#define MAX_CPUS 64
> +
> +/* Common stats data record to keep userspace more simple */
> +struct datarec {
> +       __u64 processed;
> +       __u64 dropped;
> +       __u64 issue;
> +       __u64 xdp_pass;
> +       __u64 xdp_drop;
> +       __u64 xdp_redirect;
> +};
> +
> +/* Count RX packets, as XDP bpf_prog doesn't get direct TX-success
> + * feedback.  Redirect TX errors can be caught via a tracepoint.
> + */
> +struct {
> +       __uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
> +       __type(key, u32);
> +       __type(value, struct datarec);
> +       __uint(max_entries, 1);
> +} rx_cnt SEC(".maps");
> +
> +/* Used by trace point */
> +struct {
> +       __uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
> +       __type(key, u32);
> +       __type(value, struct datarec);
> +       __uint(max_entries, 2);
> +       /* TODO: have entries for all possible errno's */
> +} redirect_err_cnt SEC(".maps");
> +
> +/* Used by trace point */
> +struct {
> +       __uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
> +       __type(key, u32);
> +       __type(value, struct datarec);
> +       __uint(max_entries, MAX_CPUS);
> +} cpumap_enqueue_cnt SEC(".maps");

One way to squeeze a bit more performance would be to instead use
global variables instead of maps:

struct datarec cpu_map_enqueue_cnts[MAX_CPUS][MAX_CPUS];

and other PERCPU_ARRAY arrays could be just one-dimensional arrays.

You'd need to ensure each value sits on its own cache-line, of course.

> +
> +/* Used by trace point */
> +struct {
> +       __uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
> +       __type(key, u32);
> +       __type(value, struct datarec);
> +       __uint(max_entries, 1);
> +} cpumap_kthread_cnt SEC(".maps");
> +

[...]

> +
> +/* Tracepoint: /sys/kernel/debug/tracing/events/xdp/xdp_cpumap_enqueue/format
> + * Code in:         kernel/include/trace/events/xdp.h
> + */
> +struct cpumap_enqueue_ctx {
> +       u64 __pad;              // First 8 bytes are not accessible by bpf code
> +       int map_id;             //      offset:8;  size:4; signed:1;
> +       u32 act;                //      offset:12; size:4; signed:0;
> +       int cpu;                //      offset:16; size:4; signed:1;
> +       unsigned int drops;     //      offset:20; size:4; signed:0;
> +       unsigned int processed; //      offset:24; size:4; signed:0;
> +       int to_cpu;             //      offset:28; size:4; signed:1;
> +};

if you used vmlinux.h, this is already in there as struct
trace_event_raw_xdp_cpumap_enqueue, similarly for other tracepoints

> +
> +SEC("tracepoint/xdp/xdp_cpumap_enqueue")
> +int trace_xdp_cpumap_enqueue(struct cpumap_enqueue_ctx *ctx)
> +{
> +       u32 to_cpu = ctx->to_cpu;
> +       struct datarec *rec;
> +
> +       if (to_cpu >= MAX_CPUS)
> +               return 1;
> +
> +       rec = bpf_map_lookup_elem(&cpumap_enqueue_cnt, &to_cpu);
> +       if (!rec)
> +               return 0;
> +       rec->processed += ctx->processed;
> +       rec->dropped   += ctx->drops;
> +
> +       /* Record bulk events, then userspace can calc average bulk size */
> +       if (ctx->processed > 0)
> +               rec->issue += 1;
> +
> +       /* Inception: It's possible to detect overload situations, via
> +        * this tracepoint.  This can be used for creating a feedback
> +        * loop to XDP, which can take appropriate actions to mitigate
> +        * this overload situation.
> +        */
> +       return 0;
> +}
> +

[...]