netdev - Re: [Patch net-next] tcp: add a tracepoint for tcp_retransmit

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Message-ID: <20171011025606.tzts5wgys5hn52p2@ast-mbp>
Date:   Tue, 10 Oct 2017 19:56:09 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:     Cong Wang <xiyou.wangcong@...il.com>, netdev@...r.kernel.org,
        Eric Dumazet <edumazet@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Martin KaFai Lau <kafai@...com>,
        Brendan Gregg <brendan.d.gregg@...il.com>,
        Song Liu <songliubraving@...com>
Subject: Re: [Patch net-next] tcp: add a tracepoint for tcp_retransmit_skb()

On Tue, Oct 10, 2017 at 11:58:53PM +0200, Hannes Frederic Sowa wrote:
> Alexei Starovoitov <alexei.starovoitov@...il.com> writes:
> 
> > On Mon, Oct 09, 2017 at 10:35:47PM -0700, Cong Wang wrote:
> 
> [...]
> 
> >> +		trace_tcp_retransmit_skb(sk, skb, segs);
> >
> > I'm happy to see new tracepoints being added to tcp stack, but I'm concerned
> > with practical usability of them.
> > Like the above tracepoint definition makes it not very useful from bpf point of view,
> > since 'sk' pointer is not recored by as part of the tracepoint.
> > In bpf/tracing world we prefer tracepoints to have raw pointers recorded
> > in TP_STRUCT__entry() and _not_ printed in TP_printk()
> > (since pointers are useless for userspace).
> 
> Ack.
> 
> Also could the TP_printk also use the socket cookies so they can get
> associated with netlink dumps and as such also be associated to user
> space processes? It could help against races while trying to associate
> the socket with a process. ss already supports dumping those cookies
> with -e.

makes sense to me.

> The corresponding commit would be:
> 
> commit 33cf7c90fe2f97afb1cadaa0cfb782cb9d1b9ee2
> Author: Eric Dumazet <edumazet@...gle.com>
> Date:   Wed Mar 11 18:53:14 2015 -0700
> 
>     net: add real socket cookies
> 
> Right now they only get set when needed but as Eric already mentioned in
> his commit log, this could be refined.

actually we hit that too for completely different tracing use case.
Indeed would be good to generate socket cookie unconditionally
for all sockets. I don't think there is any harm.