linux-kernel - Re: [PATCH v2 1/3] perf: Reorder overflow handler ahead of event

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Message-ID: <CANpmjNNi1WoE0gsq5TSv8ys68YcF2o+LtWiDj+=bZ3Y46oP7Jw@mail.gmail.com>
Date:   Thu, 7 Dec 2023 18:05:10 +0100
From:   Marco Elver <elver@...gle.com>
To:     Kyle Huey <me@...ehuey.com>
Cc:     Kyle Huey <khuey@...ehuey.com>, linux-kernel@...r.kernel.org,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Yonghong Song <yonghong.song@...ux.dev>,
        "Robert O'Callahan" <robert@...llahan.org>,
        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>,
        Ian Rogers <irogers@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        linux-perf-users@...r.kernel.org
Subject: Re: [PATCH v2 1/3] perf: Reorder overflow handler ahead of event_limit/sigtrap

On Thu, 7 Dec 2023 at 17:35, Kyle Huey <me@...ehuey.com> wrote:
>
> The perf subsystem already allows an overflow handler to clear pending_kill
> to suppress a fasync signal (although nobody currently does this). Allow an
> overflow handler to suppress the other visible side effects of an overflow,
> namely event_limit accounting and SIGTRAP generation.
>
> Signed-off-by: Kyle Huey <khuey@...ehuey.com>
> ---
>  kernel/events/core.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index b704d83a28b2..19fddfc27a4a 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -9541,6 +9541,12 @@ static int __perf_event_overflow(struct perf_event *event,
>          */
>
>         event->pending_kill = POLL_IN;
> +
> +       READ_ONCE(event->overflow_handler)(event, data, regs);
> +
> +       if (!event->pending_kill)
> +               return ret;

It's not at all intuitive that resetting pending_kill to 0 will
suppress everything else, too. There is no relationship between the
fasync signals and SIGTRAP. pending_kill is for the former and
pending_sigtrap is for the latter. One should not affect the other.

A nicer solution would be to properly undo the various pending_*
states (in the case of pending_sigtrap being set it should be enough
to reset pending_sigtrap to 0, and also decrement
event->ctx->nr_pending).

Although I can see why this solution is simpler. Perhaps with enough
comments it might be clearer.

Preferences?