Re: WARN_ON_ONCE(in_nmi()) hit in irq_work_queue_on

[Peter Zijlstra <peterz@infradead.org>]

> Sigh, that's d84153d6c96f61a so that's been there a while, and been
> broken equally long.
> 
> So this is where we run a low period (!freq) hardware event on a
> nohz_full cpu or so? And because it throttles, we need to kick the tick
> into action to unthrottle it.
> 
> I suppose there's a good reason I never build with that nohz_full
> nonsense enabled :/
> 
> Not sure how we should go fix that, you can't just issue random IPIs
> from NMI context.

OK, thinking one more second would've done it, how about so?

---
 include/linux/perf_event.h | 7 ++++---
 kernel/events/core.c       | 8 +++++++-
 2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 707617a8c0f6..177411e3ffc4 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -421,9 +421,10 @@ struct perf_event {
 	struct fasync_struct		*fasync;
 
 	/* delayed work for NMIs and such */
-	int				pending_wakeup;
-	int				pending_kill;
-	int				pending_disable;
+	int				pending_kill      : 16;
+	int				pending_wakeup    : 1;
+	int				pending_disable   : 1;
+	int				pending_nohz_kick : 1;
 	struct irq_work			pending;
 
 	atomic_t			event_limit;
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 1cf24b3e42ec..e95fca20e26f 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4258,6 +4258,11 @@ static void perf_pending_event(struct irq_work *entry)
 		event->pending_wakeup = 0;
 		perf_event_wakeup(event);
 	}
+
+	if (event->pending_nohz_kick) {
+		event->pending_nohz_kick = 0;
+		tick_nohz_full_kick();
+	}
 }
 
 /*
@@ -5431,7 +5436,8 @@ static int __perf_event_overflow(struct perf_event *event,
 			__this_cpu_inc(perf_throttled_count);
 			hwc->interrupts = MAX_INTERRUPTS;
 			perf_log_throttle(event, 0);
-			tick_nohz_full_kick();
+			event->pending_nohz_kick = 1;
+			irq_work_queue(&event->pending);
 			ret = 1;
 		}
 	}