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Date:   Thu, 22 Aug 2019 09:47:37 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Yonghong Song <yhs@fb.com>
Cc:     Daniel Xu <dxu@dxuuu.xyz>,
        "bpf@vger.kernel.org" <bpf@vger.kernel.org>,
        Song Liu <songliubraving@fb.com>,
        Andrii Nakryiko <andriin@fb.com>,
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        "acme@kernel.org" <acme@kernel.org>,
        Alexei Starovoitov <ast@fb.com>,
        "alexander.shishkin@linux.intel.com" 
        <alexander.shishkin@linux.intel.com>,
        "jolsa@redhat.com" <jolsa@redhat.com>,
        "namhyung@kernel.org" <namhyung@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Kernel Team <Kernel-team@fb.com>,
        Arnaldo Carvalho de Melo <acme@redhat.com>
Subject: Re: [PATCH v3 bpf-next 1/4] tracing/probe: Add
 PERF_EVENT_IOC_QUERY_PROBE ioctl
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On Wed, Aug 21, 2019 at 06:43:49PM +0000, Yonghong Song wrote:
> On 8/21/19 11:31 AM, Peter Zijlstra wrote:

> > So extending PERF_RECORD_LOST doesn't work. But PERF_FORMAT_LOST might
> > still work fine; but you get to implement it for all software events.
> 
> Could you give more specifics about PERF_FORMAT_LOST? Googling 
> "PERF_FORMAT_LOST" only yields two emails which we are discussing here :-(

Look at what the other PERF_FORMAT_ flags do? Basically it is adding a
field to the read(2) output.

> >> Maybe we can still use ioctl based approach which is light weighted
> >> compared to ring buffer approach? If a fd has bpf attached, nhit/nmisses
> >> means the kprobe is processed by bpf program or not.
> > 
> > There is nothing kprobe specific here. Kprobes just appear to be the
> > only one actually accounting the recursion cases, but everyone has
> > them.
> 
> Sorry to be specific, kprobe is just an example, I actually refers to 
> any perf event where bpf can attach to, which theoretically are any
> perf events which can be opened with "perf_event_open" syscall although 
> some of them (e.g., software events?) may not have bpf running hooks yet.

Yes, BPF is sucky that way.