netdev - Re: [PATCH net-next] net: add network device notifier trace points

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Date:   Wed, 19 Dec 2018 10:51:04 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Stephen Hemminger <stephen@...workplumber.org>,
        netdev@...r.kernel.org, Stephen Hemminger <sthemmin@...rosoft.com>,
        Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [PATCH net-next] net: add network device notifier trace points

On Wed, 19 Dec 2018 08:36:43 +0100
Jesper Dangaard Brouer <brouer@...hat.com> wrote:

> > +TRACE_EVENT(net_dev_notifier_entry,
> > +
> > +	TP_PROTO(const struct netdev_notifier_info *info, unsigned long val),
> > +
> > +	TP_ARGS(info, val),
> > +
> > +	TP_STRUCT__entry(
> > +		__string(	name,		 info->dev->name )
> > +		__field(	enum netdev_cmd, event	         )
> > +	),
> > +
> > +	TP_fast_assign(
> > +		__assign_str(name, info->dev->name);
> > +		__entry->event = val;
> > +       ),  
> 
> These __string and __assign_str are costly and behind the scenes does a
> strcpy.

True. But you could also make this into a memcpy with:

		__array(	char,	name,	IFNAMSIZ)


And in TP_fast_assign:

		memcpy(__entry->name, info->dev->name, IFNAMSIZ);

And for the TP_printk:

		"dev=%s", __entry->name


Yes, ifindex is still faster, but this does give you the name in a way
that I think even BPF can use it. I also believe that memcopy on a
constant is faster than a strcpy.

-- Steve