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: <20151022094902.GM2508@worktop.programming.kicks-ass.net>
Date:	Thu, 22 Oct 2015 11:49:02 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Wangnan (F)" <wangnan0@...wei.com>
Cc:	Alexei Starovoitov <ast@...mgrid.com>,
	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 Thu, Oct 22, 2015 at 11:12:16AM +0800, 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.

One could overload or stack the overflow handler I suppose. But this
would be in line with the software/tracepoint events calling eBPF muck
on trigger, right?

> >What pt_regs would be in such case?

> Sampling is based on interruption. We can use pt_reg captured by the IRQ
> handler,

s/IRQ/NMI/

Also, we 'edit' the pt_regs on the way down to the overflow handler as
sometimes 'better' information can be had from PMU state. But a pt_regs
is available there.
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