netdev - Re: [PATCH net-next 06/16] net: bridge: Add a tracepoint for MDB overflows

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Date:   Mon, 30 Jan 2023 18:23:16 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Petr Machata <petrm@...dia.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        "Paolo Abeni" <pabeni@...hat.com>, Roopa Prabhu <roopa@...dia.com>,
        "Nikolay Aleksandrov" <razor@...ckwall.org>,
        <netdev@...r.kernel.org>, <bridge@...ts.linux-foundation.org>,
        Ido Schimmel <idosch@...dia.com>,
        <linux-trace-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next 06/16] net: bridge: Add a tracepoint for MDB
 overflows

On Mon, 30 Jan 2023 16:50:32 +0100
Petr Machata <petrm@...dia.com> wrote:

> Steven Rostedt <rostedt@...dmis.org> writes:
> 
> > On Thu, 26 Jan 2023 18:01:14 +0100
> > Petr Machata <petrm@...dia.com> wrote:
> >  
> >> +	TP_printk("dev %s af %u src %pI4/%pI6c grp %pI4/%pI6c/%pM vid %u",
> >> +		  __get_str(dev), __entry->af, __entry->src4, __entry->src6,
> >> +		  __entry->grp4, __entry->grp6, __entry->grpmac, __entry->vid)  
> >
> > And just have: 
> >
> > 	TP_printk("dev %s af %u src %pI6c grp %pI6c/%pM vid %u",
> > 		  __get_str(dev), __entry->af, __entry->src, __entry->grp,
> > 		  __entry->grpmac, __entry->vid)
> >
> > As the %pI6c should detect that it's a ipv4 address and show that.  
> 
> This means the IP addresses will always be IPv6, even for an IPv4 MDB
> entries. One can still figure out the true protocol from the address
> family field, but it might not be obvious. Plus the IPv4-mapped IPv6
> addresses are not really formatted as IPv4, though yeah, IPv4 notation
> is embedded in that.
> 
> All the information is still there, but... scrambled? Not sure the
> reduction in clarity is worth the 8 bytes that we save. The tracepoint
> is unlikely to trigger often.

8 bytes per event, and yes, ring buffer real estate is expensive.

And if you use trace-cmd or perf, we can always add a plugin to
libtraceevent that can format this much nicer based on the information that
is there.

-- Steve