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

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Message-ID: <20191216191357.ftadvchztbpggcus@kafai-mbp.dhcp.thefacebook.com>
Date:   Mon, 16 Dec 2019 19:14:04 +0000
From:   Martin Lau <kafai@...com>
To:     Eric Dumazet <eric.dumazet@...il.com>
CC:     "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>,
        Kernel Team <Kernel-team@...com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next 09/13] bpf: Add BPF_FUNC_jiffies

On Fri, Dec 13, 2019 at 05:59:54PM -0800, Eric Dumazet wrote:
> 
> 
> 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.
Thanks for the feedbacks!

I have a few questions that need some helps.

Does it mean tp->tcp_mstamp can be used as the "now" in cubic?
or tcp_clock_ns() should still be called in cubic, e.g. to replace
bictcp_clock() and tcp_jiffies32?

BPF currently has a helper calling ktime_get_mono_fast_ns() which looks
different from tcp_clock_ns().

The lsndtime is in jiffies.  I think it can probably be converted to ms before
using it in cubic.  There are some BICTCP_HZ logic in bictcp_update() that
is not obvious to me how to convet them to ms base also.

> 
> 
> 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.
At least for bpf_dctcp.c, I did not expect it could be that close to tcp_dctcp.c
when I just started converted it.  tcp_cubic/bpf_cubic still has some TBD
on jiffies/msec.

Agree that it is beneficial to have one copy.   It is likely
I need to make some changes on the tcp_*.c side also.  Hence, I prefer
to give it a try in a separate series, e.g. revert the kernel side
changes will be easier.