linux-kernel - Re: [RFC PATCH tip 0/5] tracing filters with BPF

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Date:	Tue, 3 Dec 2013 10:26:18 -0800
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Tom Zanussi <tom.zanussi@...ux.intel.com>,
	Jovi Zhangwei <jovi.zhangwei@...il.com>,
	Eric Dumazet <edumazet@...gle.com>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Tom Zanussi <tzanussi@...il.com>,
	Pekka Enberg <penberg@....fi>,
	"David S. Miller" <davem@...emloft.net>,
	Arjan van de Ven <arjan@...radead.org>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [RFC PATCH tip 0/5] tracing filters with BPF

On Tue, Dec 3, 2013 at 7:33 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Tue, 3 Dec 2013 10:16:55 +0100
> Ingo Molnar <mingo@...nel.org> wrote:
>
>
>> So, to do the math:
>>
>>    tracing               'all' overhead:   95 nsecs per event
>>    tracing 'eth5 + old filter' overhead:  157 nsecs per event
>>    tracing 'eth5 + BPF filter' overhead:   54 nsecs per event
>>
>> So via BPF and a fairly trivial filter, we are able to reduce tracing
>> overhead for real - while old-style filters.
>
> Yep, seems that BPF can do what I wasn't able to do with the normal
> filters. Although, I haven't looked at the code yet, I'm assuming that
> the BPF works on the parameters passed into the trace event. The normal
> filters can only process the results of the trace (what's being
> recorded) not the parameters of the trace event itself. To get what's
> recorded, we need to write to the buffer first, and then we decided if
> we want to keep the event or not and discard the event from the buffer
> if we do not.
>
> That method does not reduce overhead at all, and only adds to it, as
> Alexei's tests have shown. The purpose of the filter was not to reduce
> overhead, but to reduce filling the buffer with needless data.

Precisely.
Assumption is that filters will filter out majority of the events.
So filter takes pt_regs as input, has to interpret them and call
bpf_trace_printk
if it really wants to store something for the human to see.
We can extend bpf trace filters to return true/false to indicate
whether TP_printk-format
specified as part of the event should be printed as well, but imo
that's unnecessary.
When I was using bpf filters to debug networking bits I didn't need
that printk format of the event. I only used event as an entry point,
filtering out things and printing different fields vs initial event.
More like what developers do when they sprinkle
trace_printk/dump_stack through the code while debugging.

the only inconvenience so far is to know how parameters are getting
into registers.
on x86-64, arg1 is in rdi, arg2 is in rsi,... I want to improve that
after first step is done.
In the proposed patches bpf_context == pt_regs at the event entry point.
Would be cleaner to have struct {arg1,arg2,…} as bpf_context instead.
But that needed more code and I wanted to keep the first patch to the
minimum.
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