linux-kernel - Re: [RFC PATCH 1/5] bpf: Put perf

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Date:	Wed, 01 Jul 2015 20:50:34 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	He Kuang <hekuang@...wei.com>, rostedt@...dmis.org,
	masami.hiramatsu.pt@...achi.com, mingo@...hat.com, acme@...hat.com,
	a.p.zijlstra@...llo.nl, jolsa@...nel.org, namhyung@...nel.org
CC:	wangnan0@...wei.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/5] bpf: Put perf_events check ahead of bpf prog

On 6/30/15 7:57 PM, He Kuang wrote:
> When we add a kprobe point and record events by perf, the execution path
> of all threads on each cpu will enter this point, but perf may only
> record events on a particular thread or cpu at this kprobe point, a
> check on call->perf_events list filters out the threads which perf is
> not recording.

I think there is a better way to do that. You're adding artificial
per_cpu filtering whereas you really need per_pid filtering.
The patch kinda worked, but looks more by accident.
The accurate way to do per_pid filtering is to automatically add
'if ((u32)bpf_get_current_pid_tgid() != expected_pid) return 0;'
as the first statement of the program.
You already have nice infra to add prologue to the program.
So I think adding above 'if' on user space side of perf is
a preferred way to achieve that.
Also such per_pid filtering should be smart.
The command 'perf record -a -e my_prog.o -- sleep 10' should
collect all things that my_prog was attached to and not only
things that happened as part of sleep.

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