Re: [PATCH] perf: Fix missing SIGTRAPs due to pending_disable abuse

[Marco Elver <elver@google.com>]

On Tue, 4 Oct 2022 at 19:09, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Wed, Sep 28, 2022 at 04:55:46PM +0200, Marco Elver wrote:
> > On Wed, Sep 28, 2022 at 12:06PM +0200, Marco Elver wrote:
> >
> > > My second idea about introducing something like irq_work_raw_sync().
> > > Maybe it's not that crazy if it is actually safe. I expect this case
> > > where we need the irq_work_raw_sync() to be very very rare.
> >
> > The previous irq_work_raw_sync() forgot about irq_work_queue_on(). Alas,
> > I might still be missing something obvious, because "it's never that
> > easy". ;-)
> >
> > And for completeness, the full perf patch of what it would look like
> > together with irq_work_raw_sync() (consider it v1.5). It's already
> > survived some shorter stress tests and fuzzing.
>
> So.... I don't like it. But I cooked up the below, which _almost_ works :-/
>
> For some raisin it sometimes fails with 14999 out of 15000 events
> delivered and I've not yet figured out where it goes sideways. I'm
> currently thinking it's that sigtrap clear on OFF.
>
> Still, what do you think of the approach?

It looks reasonable, but obviously needs to pass tests. :-)
Also, see comment below (I think you're still turning signals
asynchronous, which we shouldn't do).

> ---
>  include/linux/perf_event.h |  8 ++--
>  kernel/events/core.c       | 92 +++++++++++++++++++++++++---------------------
>  2 files changed, 55 insertions(+), 45 deletions(-)
>
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index ee8b9ecdc03b..c54161719d37 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -736,9 +736,11 @@ struct perf_event {
>         struct fasync_struct            *fasync;
>
>         /* delayed work for NMIs and such */
> -       int                             pending_wakeup;
> -       int                             pending_kill;
> -       int                             pending_disable;
> +       unsigned int                    pending_wakeup  :1;
> +       unsigned int                    pending_disable :1;
> +       unsigned int                    pending_sigtrap :1;
> +       unsigned int                    pending_kill    :3;
> +
>         unsigned long                   pending_addr;   /* SIGTRAP */
>         struct irq_work                 pending;
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 2621fd24ad26..8e5dbe971d9e 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -2268,11 +2268,15 @@ event_sched_out(struct perf_event *event,
>         event->pmu->del(event, 0);
>         event->oncpu = -1;
>
> -       if (READ_ONCE(event->pending_disable) >= 0) {
> -               WRITE_ONCE(event->pending_disable, -1);
> +       if (event->pending_disable) {
> +               event->pending_disable = 0;
>                 perf_cgroup_event_disable(event, ctx);
>                 state = PERF_EVENT_STATE_OFF;
>         }
> +
> +       if (event->pending_sigtrap && state == PERF_EVENT_STATE_OFF)
> +               event->pending_sigtrap = 0;
> +
>         perf_event_set_state(event, state);
>
>         if (!is_software_event(event))
> @@ -2463,8 +2467,7 @@ EXPORT_SYMBOL_GPL(perf_event_disable);
>
>  void perf_event_disable_inatomic(struct perf_event *event)
>  {
> -       WRITE_ONCE(event->pending_disable, smp_processor_id());
> -       /* can fail, see perf_pending_event_disable() */
> +       event->pending_disable = 1;
>         irq_work_queue(&event->pending);
>  }
>
> @@ -2527,6 +2530,9 @@ event_sched_in(struct perf_event *event,
>         if (event->attr.exclusive)
>                 cpuctx->exclusive = 1;
>
> +       if (event->pending_disable || event->pending_sigtrap)
> +               irq_work_queue(&event->pending);
> +
>  out:
>         perf_pmu_enable(event->pmu);
>
> @@ -6440,47 +6446,40 @@ static void perf_sigtrap(struct perf_event *event)
>                       event->attr.type, event->attr.sig_data);
>  }
>
> -static void perf_pending_event_disable(struct perf_event *event)
> +/*
> + * Deliver the pending work in-event-context or follow the context.
> + */
> +static void __perf_pending_event(struct perf_event *event)
>  {
> -       int cpu = READ_ONCE(event->pending_disable);
> +       int cpu = READ_ONCE(event->oncpu);
>
> +       /*
> +        * If the event isn't running; we done. event_sched_in() will restart
> +        * the irq_work when needed.
> +        */
>         if (cpu < 0)
>                 return;
>
> +       /*
> +        * Yay, we hit home and are in the context of the event.
> +        */
>         if (cpu == smp_processor_id()) {
> -               WRITE_ONCE(event->pending_disable, -1);
> -
> -               if (event->attr.sigtrap) {
> +               if (event->pending_sigtrap) {
> +                       event->pending_sigtrap = 0;
>                         perf_sigtrap(event);
> -                       atomic_set_release(&event->event_limit, 1); /* rearm event */
> -                       return;
>                 }
> -
> -               perf_event_disable_local(event);
> -               return;
> +               if (event->pending_disable) {
> +                       event->pending_disable = 0;
> +                       perf_event_disable_local(event);
> +               }
>         }
>
>         /*
> -        *  CPU-A                       CPU-B
> -        *
> -        *  perf_event_disable_inatomic()
> -        *    @pending_disable = CPU-A;
> -        *    irq_work_queue();
> -        *
> -        *  sched-out
> -        *    @pending_disable = -1;
> -        *
> -        *                              sched-in
> -        *                              perf_event_disable_inatomic()
> -        *                                @pending_disable = CPU-B;
> -        *                                irq_work_queue(); // FAILS
> -        *
> -        *  irq_work_run()
> -        *    perf_pending_event()
> -        *
> -        * But the event runs on CPU-B and wants disabling there.
> +        * Requeue if there's still any pending work left, make sure to follow
> +        * where the event went.
>          */
> -       irq_work_queue_on(&event->pending, cpu);
> +       if (event->pending_disable || event->pending_sigtrap)
> +               irq_work_queue_on(&event->pending, cpu);

I considered making the irq_work "chase" the right CPU but it doesn't
work for sigtrap. This will make the signal asynchronous (it should be
synchronous), and the reason why I had to do irq_work_raw_sync().

Thanks,
-- Marco