netdev - Re: [RFC PATCH bpf-next 2/4] bpf: introduce BPF dispatcher

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Date:   Wed, 13 Nov 2019 21:40:28 +0000
From:   Edward Cree <ecree@...arflare.com>
To:     Björn Töpel <bjorn.topel@...il.com>,
        <netdev@...r.kernel.org>, <ast@...nel.org>, <daniel@...earbox.net>
CC:     Björn Töpel <bjorn.topel@...el.com>,
        <bpf@...r.kernel.org>, <magnus.karlsson@...il.com>,
        <magnus.karlsson@...el.com>, <jonathan.lemon@...il.com>
Subject: Re: [RFC PATCH bpf-next 2/4] bpf: introduce BPF dispatcher

On 13/11/2019 20:47, Björn Töpel wrote:
> From: Björn Töpel <bjorn.topel@...el.com>
> 
> The BPF dispatcher builds on top of the BPF trampoline ideas;
> Introduce bpf_arch_text_poke() and (re-)use the BPF JIT generate
> code. The dispatcher builds a dispatch table for XDP programs, for
> retpoline avoidance. The table is a simple binary search model, so
> lookup is O(log n). Here, the dispatch table is limited to four
> entries (for laziness reason -- only 1B relative jumps :-P). If the
> dispatch table is full, it will fallback to the retpoline path.
> 
> An example: A module/driver allocates a dispatcher. The dispatcher is
> shared for all netdevs. Each netdev allocate a slot in the dispatcher
> and a BPF program. The netdev then uses the dispatcher to call the
> correct program with a direct call (actually a tail-call).
> 
> Signed-off-by: Björn Töpel <bjorn.topel@...el.com>
The first-come-first-served model for dispatcher slots might mean that
 a low-traffic user ends up getting priority while a higher-traffic
 user is stuck with the retpoline fallback.  Have you considered using
 a learning mechanism, like in my dynamic call RFC [1] earlier this
 year?  (Though I'm sure a better learning mechanism than the one I
 used there could be devised.)

> +static int bpf_dispatcher_add_prog(struct bpf_dispatcher *d,
> +				   struct bpf_prog *prog)
> +{
> +	struct bpf_prog **entry = NULL;
> +	int i, err = 0;
> +
> +	if (d->num_progs == BPF_DISPATCHER_MAX)
> +		return err;
> +
> +	for (i = 0; i < BPF_DISPATCHER_MAX; i++) {
> +		if (!entry && !d->progs[i])
> +			entry = &d->progs[i];
> +		if (d->progs[i] == prog)
> +			return err;
> +	}
> +
> +	prog = bpf_prog_inc(prog);
> +	if (IS_ERR(prog))
> +		return err;
> +
> +	*entry = prog;
> +	d->num_progs++;
> +	return err;
> +}
If I'm reading this function right, it always returns zero; was that
 the intention, and if so why isn't it void?

-Ed

[1] https://lkml.org/lkml/2019/2/1/948