linux-kernel - Re: [PATCH V5 1/1] bpf: control events stored in PERF_EVENT_ARRAY maps trace data output when perf sampling

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Message-ID: <5628577D.4030300@plumgrid.com>
Date:	Wed, 21 Oct 2015 20:26:53 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	"Wangnan (F)" <wangnan0@...wei.com>,
	Peter Zijlstra <peterz@...radead.org>
Cc:	xiakaixu <xiakaixu@...wei.com>, davem@...emloft.net,
	acme@...nel.org, mingo@...hat.com, masami.hiramatsu.pt@...achi.com,
	jolsa@...nel.org, daniel@...earbox.net,
	linux-kernel@...r.kernel.org, pi3orama@....com, hekuang@...wei.com,
	netdev@...r.kernel.org
Subject: Re: [PATCH V5 1/1] bpf: control events stored in PERF_EVENT_ARRAY
 maps trace data output when perf sampling

On 10/21/15 8:12 PM, Wangnan (F) wrote:
>
>
> On 2015/10/22 11:09, Alexei Starovoitov wrote:
>> On 10/21/15 6:56 PM, Wangnan (F) wrote:
>>>> One alternative solution I can image is to attach a BPF program
>>>> at sampling like kprobe, and return 0 if we don't want sampling
>>>> take action. Thought?
>>>
>>> Do you think attaching BPF programs to sampling is an acceptable idea?
>>
>> If you mean to extend 'filter' concept to sampling events?
>> So instead of soft_disable of non-local events, you'll attach bpf
>> program to sampling events and use map lookup to decide whether
>> to filter out or not such sampling event?
>
> Yes.
>
>> What pt_regs would be in such case?
>>
>
> Sampling is based on interruption. We can use pt_reg captured by the IRQ
> handler,
> or we can simply pass NULL to those BPF program.

NULL is obviously not ok. Try to answer yourself 'why it's not'.
Clean implementation should add single 'if (..->prog)' to event sampling
critical path. Please don't rush it and think it through.
but the first thing first, please see my fix for bpf_perf_event_read:
http://patchwork.ozlabs.org/patch/534120/
and ack it if it makes sense.
we need to make sure existing stuff is solid, before going further.

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