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From: Steven Rostedt on 6 Aug 2010 16:40
On Wed, 2010-07-28 at 00:44 +0200, Marcin Slusarz wrote:
> 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(a)gmail.com>
> Cc: Steven Rostedt <rostedt(a)goodmis.org>
> Cc: Frederic Weisbecker <fweisbec(a)gmail.com>
> Cc: Ingo Molnar <mingo(a)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();
> }
>

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


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From: Steven Rostedt on 6 Aug 2010 17:30
On Fri, 2010-08-06 at 22:55 +0200, Frederic Weisbecker wrote:

> > 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.
>
>
> 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);
>

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
like the scheduler or NMI.


> 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.

Yep, will put that on my todo list.

Thanks,

-- Steve


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