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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Tue, 26 Apr 2022 16:55:17 -0700
Message-ID: <CAEf4Bzbdh-wbQQLzoXGGKkqqE=+qz19C4tCq4Ynb-_PXzRYM1w@mail.gmail.com>
Subject: Re: [PATCH 4/4] perf record: Handle argument change in sched_switch
To:     Namhyung Kim <namhyung@kernel.org>
Cc:     Arnaldo Carvalho de Melo <acme@kernel.org>,
        Jiri Olsa <jolsa@kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Andi Kleen <ak@linux.intel.com>,
        Ian Rogers <irogers@google.com>,
        Song Liu <songliubraving@fb.com>, Hao Luo <haoluo@google.com>,
        bpf <bpf@vger.kernel.org>,
        "linux-perf-use." <linux-perf-users@vger.kernel.org>,
        Blake Jones <blakejones@google.com>
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On Fri, Apr 22, 2022 at 3:49 PM Namhyung Kim <namhyung@kernel.org> wrote:
>
> Recently sched_switch tracepoint added a new argument for prev_state,
> but it's hard to handle the change in a BPF program.  Instead, we can
> check the function prototype in BTF before loading the program.
>
> Thus I make two copies of the tracepoint handler and select one based
> on the BTF info.
>
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> ---
>  tools/perf/util/bpf_off_cpu.c          | 32 +++++++++++++++
>  tools/perf/util/bpf_skel/off_cpu.bpf.c | 55 ++++++++++++++++++++------
>  2 files changed, 76 insertions(+), 11 deletions(-)
>

[...]

>
> +SEC("tp_btf/sched_switch")
> +int on_switch3(u64 *ctx)
> +{
> +       struct task_struct *prev, *next;
> +       int state;
> +
> +       if (!enabled)
> +               return 0;
> +
> +       /*
> +        * TP_PROTO(bool preempt, struct task_struct *prev,
> +        *          struct task_struct *next)
> +        */
> +       prev = (struct task_struct *)ctx[1];
> +       next = (struct task_struct *)ctx[2];


you don't have to have two BPF programs for this, you can use
read-only variable to make this choice.

On BPF side

const volatile bool has_prev_state = false;

...

if (has_prev_state) {
    prev = (struct task_struct *)ctx[2];
    next = (struct task_struct *)ctx[3];
} else {
    prev = (struct task_struct *)ctx[1];
    next = (struct task_struct *)ctx[2];
}


And from user-space side you do your detection and before skeleton is loaded:

skel->rodata->has_prev_state = <whatever you detected>

But I'm still hoping that this prev_state argument can be moved to the
end ([0]) to make all this unnecessary.

  [0] https://lore.kernel.org/lkml/93a20759600c05b6d9e4359a1517c88e06b44834.camel@fb.com/


> +
> +       state = get_task_state(prev);
> +
> +       return on_switch(ctx, prev, next, state);
> +}
> +

[...]