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

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Date:	Wed, 30 Jul 2014 11:53:54 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	"Frank Ch. Eigler" <fche@...hat.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 <linux-api@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v3 net-next 3/3] samples: bpf: eBPF dropmon example in C

On Wed, Jul 30, 2014 at 10:36 AM, Frank Ch. Eigler <fche@...hat.com> wrote:
>> > What kind of locking/serialization is provided by the ebpf runtime
>> > over shared variables such as my_map?
>>
>> it's traditional rcu scheme.
>
> OK, that protects the table structure, but:
>
>> [...] In such case concurrent write access to map value can be done
>> with bpf_xadd instruction, though using normal read/write is also
>> allowed. In some cases the speed of racy var++ is preferred over
>> 'lock xadd'.
>
> ... so concurrency control over shared values is left up to the
> programmer.

yes. It has to be flexible and fast.
One of our main use cases is network analytics where a lot of
packets are going through ebpf programs, so every cycle counts.
Mandatory locks in critical path are not acceptable. If we add
locks they will be optional.

>> There are no lock/unlock function helpers available to ebpf
>> programs, since program may terminate early with div by zero
>> for example, so in-kernel lock helper implementation would
>> be complicated and slow. It's possible to do, but for the use
>> cases so far there is no need.
>
> OK, I hope that works out.  I've been told that dtrace does something
> similiar (!)  by eschewing protection on global variables such as
> strings.  In their case it's less bad than it sounds because they are
> used to offloading computation to userspace or to store only
> thread-local state, and accept the corollary limitations on control.

interesting.
btw, things like global variables, per-cpu storage are potential ebpf
features. So far they're 'nice to have' instead of 'mandatory'.
The maps are powerful enough to do the same:
Global storage is map of one element.
Per-cpu storage is map of num_cpu elements.
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