linux-kernel - Re: [PATCH] perf/core: Trim dyn

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Date: Mon, 29 Apr 2024 14:59:23 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Yabin Cui <yabinc@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>, 
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org, 
	bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH] perf/core: Trim dyn_size if raw data is absent

Hello Yabin,

CC-ing the bpf list.

On Thu, Apr 25, 2024 at 3:05 PM Yabin Cui <yabinc@...gle.com> wrote:
>
> Currently, perf_tp_event() always allocates space for raw sample data,
> even when the PERF_SAMPLE_RAW flag is not set. This leads to unused
> spaces within generated sample records.
>
> This patch reduces dyn_size when PERF_SAMPLE_RAW is not present,
> ensuring sample records use only the necessary amount of space.

Right, it seems bpf-output and tracepoint events set the flags without
checking PERF_SAMPLE_RAW.  Can you fix the callsites instead?
Or we can add perf_event argument to perf_sample_save_raw_data()
and check the flag inside.

We might reject the output data when it's not opened with the flag.
But I'm afraid it might break some existing BPF programs.

Thanks,
Namhyung

>
> Fixes: 0a9081cf0a11 ("perf/core: Add perf_sample_save_raw_data() helper")
> Signed-off-by: Yabin Cui <yabinc@...gle.com>
> ---
>  kernel/events/core.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 724e6d7e128f..d68ecdc264d3 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -7688,6 +7688,10 @@ void perf_prepare_sample(struct perf_sample_data *data,
>                 data->raw = NULL;
>                 data->dyn_size += sizeof(u64);
>                 data->sample_flags |= PERF_SAMPLE_RAW;
> +       } else if ((data->sample_flags & ~sample_type) & PERF_SAMPLE_RAW) {
> +               data->dyn_size -= data->raw->size + sizeof(u32);
> +               data->raw = NULL;
> +               data->sample_flags &= ~PERF_SAMPLE_RAW;
>         }
>
>         if (filtered_sample_type & PERF_SAMPLE_BRANCH_STACK) {
> --
> 2.44.0.769.g3c40516874-goog
>