netdev - Re: [PATCH bpf-next 09/13] bpf: Add BPF_FUNC

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Date:   Fri, 13 Dec 2019 17:59:54 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Martin KaFai Lau <kafai@...com>, bpf@...r.kernel.org
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>, kernel-team@...com,
        netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next 09/13] bpf: Add BPF_FUNC_jiffies



On 12/13/19 4:47 PM, Martin KaFai Lau wrote:
> This patch adds a helper to handle jiffies.  Some of the
> tcp_sock's timing is stored in jiffies.  Although things
> could be deduced by CONFIG_HZ, having an easy way to get
> jiffies will make the later bpf-tcp-cc implementation easier.
> 

...

> +
> +BPF_CALL_2(bpf_jiffies, u64, in, u64, flags)
> +{
> +	if (!flags)
> +		return get_jiffies_64();
> +
> +	if (flags & BPF_F_NS_TO_JIFFIES) {
> +		return nsecs_to_jiffies(in);
> +	} else if (flags & BPF_F_JIFFIES_TO_NS) {
> +		if (!in)
> +			in = get_jiffies_64();
> +		return jiffies_to_nsecs(in);
> +	}
> +
> +	return 0;
> +}

This looks a bit convoluted :)

Note that we could possibly change net/ipv4/tcp_cubic.c to no longer use jiffies at all.

We have in tp->tcp_mstamp an accurate timestamp (in usec) that can be converted to ms.


Have you thought of finding a way to not duplicate the code for cubic and dctcp, maybe
by including a template ?

Maintaining two copies means that future changes need more maintenance work.