On Thu, Feb 13, 2020 at 09:29:51AM +0100, Peter Zijlstra wrote:

On Wed, Feb 12, 2020 at 06:28:30PM -0500, Joel Fernandes wrote:

quoted

On Wed, Feb 12, 2020 at 10:01:45PM +0100, Peter Zijlstra wrote:

quoted

The tracepoint interface will stop providing regular RCU context; make
sure we do it ourselves, since perf makes use of regular RCU protected
data.

Suggested-by: Steven Rostedt (VMware) <redacted>
Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/events/core.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/kernel/events/core.c
+++ b/kernel/events/core.c

@@ -8950,6 +8950,7 @@ void perf_tp_event(u16 event_type, u64 c
 {
 	struct perf_sample_data data;
 	struct perf_event *event;
+	unsigned long rcu_flags;

The flags are not needed I guess, if you agree on not using in_nmi() in
trace_rcu_enter().

Even then we need to store the state: 'didn't do nothing' vs 'did call
rcu_needs_to_wake_up_and_pay_attention_noaw'. That is, we only need to
do something (expensive!) when !rcu_is_watching().

You are right, that sounds good. I was talking to Paul and we chatted that if
in_nmi() is safe (which I believe it is as we are not calling RCU before you
update the preempt counts), then in RCU we can replace the @irq with
!in_nmi() and simplify that code.  Then we can simplify this bit as well
(keep rcu_flags but only call rcu_irq_enter_irqsave() instead of
rcu_nmi_enter(). May be you can do the RCU internal bits in your v3 or should
those be separate patches? Whatever Paul and you want to do.

thanks,

 - Joel

 - Joel