Re: [PATCH v2 1/3] perf: Reorder overflow handler ahead of event_limit/sigtrap

[Namhyung Kim <namhyung@kernel.org>]

Hello,

On Thu, Dec 07, 2023 at 06:53:58PM +0100, Marco Elver wrote:
> On Thu, 7 Dec 2023 at 18:47, Kyle Huey <me@kylehuey.com> wrote:
> >
> >
> >
> > On Thu, Dec 7, 2023, 9:05 AM Marco Elver <elver@google.com> wrote:
> >>
> >> On Thu, 7 Dec 2023 at 17:35, Kyle Huey <me@kylehuey.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.

Well, I think it can still hit the throttling logic and generate
a PERF_RECORD_THROTTLE.  But it should be rare..

> >> >
> >> > Signed-off-by: Kyle Huey <khuey@kylehuey.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).
> >
> >
> > I don't believe it's possible to correctly undo the event_limit decrement after the fact (if it's e.g. racing with the ioctl that adds to the event limit).
> >
> >> Although I can see why this solution is simpler. Perhaps with enough
> >> comments it might be clearer.
> >>
> >> Preferences?
> >
> >
> > The cleanest way would probably be to add a return value to the overflow handler function that controls this. It requires changing a bunch of arch specific code on arches I don't have access to though.
> 
> Hmm.
> 
> Maybe wait for perf maintainers to say what is preferrable. (I could
> live with just making sure this has no other weird side effects and
> more comments.)

What if we can call bpf handler directly and check the return value?
Then I think we can also get rid of the original overflow handler.

Something like this (untested..)

Thanks,
Namhyung


---8<---

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index e85cd1c0eaf3..1eba6f5bb70b 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -809,7 +809,6 @@ struct perf_event {
 	perf_overflow_handler_t		overflow_handler;
 	void				*overflow_handler_context;
 #ifdef CONFIG_BPF_SYSCALL
-	perf_overflow_handler_t		orig_overflow_handler;
 	struct bpf_prog			*prog;
 	u64				bpf_cookie;
 #endif
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 4c72a41f11af..e1a00646dbbe 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -9471,6 +9471,12 @@ static inline bool sample_is_allowed(struct perf_event *event, struct pt_regs *r
  * Generic event overflow handling, sampling.
  */
 
+#ifdef CONFIG_BPF_SYSCALL
+static int bpf_overflow_handler(struct perf_event *event,
+				struct perf_sample_data *data,
+				struct pt_regs *regs);
+#endif
+
 static int __perf_event_overflow(struct perf_event *event,
 				 int throttle, struct perf_sample_data *data,
 				 struct pt_regs *regs)
@@ -9487,6 +9493,11 @@ static int __perf_event_overflow(struct perf_event *event,
 
 	ret = __perf_event_account_interrupt(event, throttle);
 
+#ifdef CONFIG_BPF_SYSCALL
+	if (event->prog && bpf_overflow_handler(event, data, regs) == 0)
+		return ret;
+#endif
+
 	/*
 	 * XXX event_limit might not quite work as expected on inherited
 	 * events
@@ -10346,7 +10357,7 @@ static void perf_event_free_filter(struct perf_event *event)
 }
 
 #ifdef CONFIG_BPF_SYSCALL
-static void bpf_overflow_handler(struct perf_event *event,
+static int bpf_overflow_handler(struct perf_event *event,
 				 struct perf_sample_data *data,
 				 struct pt_regs *regs)
 {
@@ -10355,7 +10366,7 @@ static void bpf_overflow_handler(struct perf_event *event,
 		.event = event,
 	};
 	struct bpf_prog *prog;
-	int ret = 0;
+	int ret = 1;
 
 	ctx.regs = perf_arch_bpf_user_pt_regs(regs);
 	if (unlikely(__this_cpu_inc_return(bpf_prog_active) != 1))
@@ -10369,10 +10380,7 @@ static void bpf_overflow_handler(struct perf_event *event,
 	rcu_read_unlock();
 out:
 	__this_cpu_dec(bpf_prog_active);
-	if (!ret)
-		return;
-
-	event->orig_overflow_handler(event, data, regs);
+	return ret;
 }
 
 static int perf_event_set_bpf_handler(struct perf_event *event,
@@ -10408,8 +10416,6 @@ static int perf_event_set_bpf_handler(struct perf_event *event,
 
 	event->prog = prog;
 	event->bpf_cookie = bpf_cookie;
-	event->orig_overflow_handler = READ_ONCE(event->overflow_handler);
-	WRITE_ONCE(event->overflow_handler, bpf_overflow_handler);
 	return 0;
 }
 
@@ -10420,7 +10426,6 @@ static void perf_event_free_bpf_handler(struct perf_event *event)
 	if (!prog)
 		return;
 
-	WRITE_ONCE(event->overflow_handler, event->orig_overflow_handler);
 	event->prog = NULL;
 	bpf_prog_put(prog);
 }
@@ -11880,13 +11885,11 @@ perf_event_alloc(struct perf_event_attr *attr, int cpu,
 		overflow_handler = parent_event->overflow_handler;
 		context = parent_event->overflow_handler_context;
 #if defined(CONFIG_BPF_SYSCALL) && defined(CONFIG_EVENT_TRACING)
-		if (overflow_handler == bpf_overflow_handler) {
+		if (parent_event->prog) {
 			struct bpf_prog *prog = parent_event->prog;
 
 			bpf_prog_inc(prog);
 			event->prog = prog;
-			event->orig_overflow_handler =
-				parent_event->orig_overflow_handler;
 		}
 #endif
 	}