Currently we rely on other code periodically waking up trace reader.
If there aren't any other data than markers, reader will never be woken up.
Fix it.

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
---
 kernel/trace/trace.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 086d363..02e04c8 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -1520,6 +1520,7 @@ int trace_array_vprintk(struct trace_array *tr,
 	if (!filter_check_discard(call, entry, buffer, event)) {
 		ring_buffer_unlock_commit(buffer, event);
 		ftrace_trace_stack(buffer, irq_flags, 6, pc);
+		trace_wake_up();
 	}
 
  out_unlock:
-- 
1.7.1.1

--

[ message continues ]

This can't work. trace_printk() and friends must be able to be used
anywhere. This can cause race conditions with the rq locks in the
scheduler.

But you do bring up a good idea. That is, perhaps we should have a way
to attach to known safe tracepoints that we can hook to to check if a
wake up should happen or not.

-- Steve


--

[ message continues ]

This could be a simple macro that takes the name of the trace event:


DEFINE_EVENT(event_tpl, event_name, ...);


TRACE_EVENT_NO_WAKE(event_name);

I think trace events should be wakeable by default as it looks safe for
most of them. But probably we don't want that per event class.

In the unsafe list, I only have some sched and lock events in
mind, but I bet there are some others.

--

[ message continues ]

Yeah, that may be worth doing for 2.6.37. Might as well also add a
trace_printk_nowake() too, when you know you are in dangerous locations

Yep, will put that on my todo list.

Thanks,

-- Steve


--

[ message continues ]

Cool. This is going to be useful in perf as well. The "nmi" argument in
perf_swevent_add tells wether we can wake up or not. If not we do a
kind of delayed wake up using a self IPI.

Currently we always consider we can't wake up when a trace event triggers.
If we know we can wake up, this is going to be less costly.

--

[ message continues ]