linux-kernel - Re: [RFC PATCH 0/6] bpf: Introduce the new ability of eBPF programs to access hardware PMU counter

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Message-ID: <55A9AA4E.6040404@plumgrid.com>
Date:	Fri, 17 Jul 2015 18:22:22 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	pi3orama <pi3orama@....com>
Cc:	kaixu xia <xiakaixu@...wei.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"acme@...nel.org" <acme@...nel.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"a.p.zijlstra@...llo.nl" <a.p.zijlstra@...llo.nl>,
	"masami.hiramatsu.pt@...achi.com" <masami.hiramatsu.pt@...achi.com>,
	"jolsa@...nel.org" <jolsa@...nel.org>,
	"wangnan0@...wei.com" <wangnan0@...wei.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hekuang@...wei.com" <hekuang@...wei.com>
Subject: Re: [RFC PATCH 0/6] bpf: Introduce the new ability of eBPF programs
 to access hardware PMU counter

On 7/17/15 6:02 PM, pi3orama wrote:
> An example: we want to count the number of cycles between entry and exit point of a particular library function (glibc write() for example). Context switch is possible, but we don't care cycles consumed by other tasks. Then we need to create a perf event in task context using:
>
> perf _event_open(&attr, pid, -1/* cpu */, ...);
> Since it is a library function, we have to choose pids we interest.

sure. just store that fd under whatever index in perf_event_array
and use it from the program. index is not cpuid. it's just an index.

> We should also probe sys_clone and create new perf event when thread creating, we haven't think how to do that now.

opening a perf_event from the program? That will be very very hard.
Much easier to kprobe sys_clone and signal to user space via
bpf_output_trace_data() and user space will be
perf_event_open-ing new event for new task.

ps
please tweak your email client to wrap lines.

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