Re: [PATCH RFC 3/4] perf/core: Add support for SIGTRAP on perf events

[Dmitry Vyukov]

On Tue, Feb 23, 2021 at 3:34 PM Marco Elver <elver@xxxxxxxxxx> wrote:
>
> Adds bit perf_event_attr::sigtrap, which can be set to cause events to
> send SIGTRAP (with si_code TRAP_PERF) to the task where the event
> occurred. To distinguish perf events and allow user space to decode
> si_perf (if set), the event type is set in si_errno.
>
> The primary motivation is to support synchronous signals on perf events
> in the task where an event (such as breakpoints) triggered.
>
> Link: https://lore.kernel.org/lkml/YBv3rAT566k+6zjg@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
> ---
>  include/uapi/linux/perf_event.h |  3 ++-
>  kernel/events/core.c            | 21 +++++++++++++++++++++
>  2 files changed, 23 insertions(+), 1 deletion(-)
>
> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index ad15e40d7f5d..b9cc6829a40c 100644
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -389,7 +389,8 @@ struct perf_event_attr {
>                                 cgroup         :  1, /* include cgroup events */
>                                 text_poke      :  1, /* include text poke events */
>                                 build_id       :  1, /* use build id in mmap2 events */
> -                               __reserved_1   : 29;
> +                               sigtrap        :  1, /* send synchronous SIGTRAP on event */
> +                               __reserved_1   : 28;
>
>         union {
>                 __u32           wakeup_events;    /* wakeup every n events */
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 37a8297be164..8718763045fd 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -6288,6 +6288,17 @@ void perf_event_wakeup(struct perf_event *event)
>         }
>  }
>
> +static void perf_sigtrap(struct perf_event *event)
> +{
> +       struct kernel_siginfo info;
> +
> +       clear_siginfo(&info);
> +       info.si_signo = SIGTRAP;
> +       info.si_code = TRAP_PERF;
> +       info.si_errno = event->attr.type;
> +       force_sig_info(&info);
> +}
> +
>  static void perf_pending_event_disable(struct perf_event *event)
>  {
>         int cpu = READ_ONCE(event->pending_disable);
> @@ -6297,6 +6308,13 @@ static void perf_pending_event_disable(struct perf_event *event)
>
>         if (cpu == smp_processor_id()) {
>                 WRITE_ONCE(event->pending_disable, -1);
> +
> +               if (event->attr.sigtrap) {
> +                       atomic_inc(&event->event_limit); /* rearm event */

Can/should this be atomic_set(&event->event_limit, 1)? It should only
go between 1 and 0, right?
Otherwise:

Acked-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>


> +                       perf_sigtrap(event);
> +                       return;
> +               }
> +
>                 perf_event_disable_local(event);
>                 return;
>         }
> @@ -11325,6 +11343,9 @@ perf_event_alloc(struct perf_event_attr *attr, int cpu,
>
>         event->state            = PERF_EVENT_STATE_INACTIVE;
>
> +       if (event->attr.sigtrap)
> +               atomic_set(&event->event_limit, 1);
> +
>         if (task) {
>                 event->attach_state = PERF_ATTACH_TASK;
>                 /*
> --
> 2.30.0.617.g56c4b15f3c-goog
>