linux-kernel - Re: [PATCH RFC v3 net-next 3/3] samples: bpf: eBPF dropmon example in C

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Date:	Wed, 30 Jul 2014 11:45:28 -0400
From:	fche@...hat.com (Frank Ch. Eigler)
To:	Alexei Starovoitov <ast@...mgrid.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Steven Rostedt <rostedt@...dmis.org>,
	Daniel Borkmann <dborkman@...hat.com>,
	Chema Gonzalez <chema@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Jiri Olsa <jolsa@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>, linux-api@...r.kernel.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC v3 net-next 3/3] samples: bpf: eBPF dropmon example in C


ast wrote earlier:

> [...]
> dtrace/systemtap/ktap approach is to use one script file that should provide
> all desired functionality. That architectural decision overcomplicated their
> implementations.
>
> eBPF follows split model: everything that needs to process millions of events
> per second needs to run in kernel and needs to be short and deterministic,
> all other things like aggregation and nice graphs should run in user space.
> [...]

For the record, this is not entirely accurate as to dtrace.  dtrace
delegates aggregation and most reporting to userspace.  Also,
systemtap is "short and deterministic" even for aggregations & nice
graphs, but since it limits its storage & cpu consumption, its
arrays/reports cannot get super large.


> [...]
> +SEC("events/skb/kfree_skb")
> +int bpf_prog2(struct bpf_context *ctx)
> +{
> +[...]
> +	value = bpf_map_lookup_elem(&my_map, &loc);
> +	if (value)
> +		(*(long *) value) += 1;
> +	else
> +		bpf_map_update_elem(&my_map, &loc, &init_val);
> +	return 0;
> +}

What kind of locking/serialization is provided by the ebpf runtime
over shared variables such as my_map?


- FChE
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