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From:   Jiri Olsa <olsajiri@gmail.com>
Date:   Fri, 23 Dec 2022 08:53:33 +0100
To:     Namhyung Kim <namhyung@kernel.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Alexei Starovoitov <ast@kernel.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Song Liu <songliubraving@fb.com>,
        Martin KaFai Lau <kafai@fb.com>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>, Hao Luo <haoluo@google.com>,
        Stanislav Fomichev <sdf@google.com>,
        LKML <linux-kernel@vger.kernel.org>, bpf@vger.kernel.org,
        Ingo Molnar <mingo@kernel.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>
Subject: Re: [PATCH bpf-next 1/2] bpf/perf: Call perf_prepare_sample() before
 bpf_prog_run()
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On Thu, Dec 22, 2022 at 02:25:49PM -0800, Namhyung Kim wrote:
> On Thu, Dec 22, 2022 at 12:16 PM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Thu, Dec 22, 2022 at 09:34:42AM -0800, Namhyung Kim wrote:
> >
> > > Sorry about that.  Let me rephrase it like below:
> > >
> > > With bpf_cast_to_kern_ctx(), BPF programs attached to a perf event
> > > can access perf sample data directly from the ctx.
> >
> > This is the bpf_prog_run() in bpf_overflow_handler(), right?
> 
> Yes.
> 
> >
> > > But the perf sample
> > > data is not fully prepared at this point, and some fields can have invalid
> > > uninitialized values.  So it needs to call perf_prepare_sample() before
> > > calling the BPF overflow handler.
> >
> > It never was, why is it a problem now?
> 
> BPF used to allow selected fields only like period and addr, and they
> are initialized always by perf_sample_data_init().  This is relaxed
> by the bpf_cast_to_kern_ctx() and it can easily access arbitrary
> fields of perf_sample_data now.
> 
> The background of this change is to use BPF as a filter for perf
> event samples.  The code is there already and returning 0 from
> BPF can drop perf samples.  With access to more sample data,
> it'd make more educated decisions.
> 
> For example, I got some requests to limit perf samples in a
> selected region of address (code or data).  Or it can collect
> samples only if some hardware specific information is set in
> the raw data like in AMD IBS.  We can easily extend it to other
> sample info based on users' needs.
> 
> >
> > > But just calling perf_prepare_sample() can be costly when the BPF
> >
> > So you potentially call it twice now, how's that useful?
> 
> Right.  I think we can check data->sample_flags in
> perf_prepare_sample() to minimize the duplicate work.
> It already does it for some fields, but misses others.

we used to have __PERF_SAMPLE_CALLCHAIN_EARLY to avoid extra perf_callchain,
could we add some flag like __PERF_SAMPLE_INIT_EARLY to avoid double call to
perf_prepare_sample?

jirka 

> 
> Thanks,
> Namhyung