linux-kernel - Re: [PATCH] tracing: wake up tasks reading trace_pipe on write to trace

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Date:	Fri, 6 Aug 2010 22:55:51 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Marcin Slusarz <marcin.slusarz@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] tracing: wake up tasks reading trace_pipe on write to
	trace_marker

On Fri, Aug 06, 2010 at 04:38:56PM -0400, Steven Rostedt wrote:
> 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@...il.com>
> > Cc: Steven Rostedt <rostedt@...dmis.org>
> > Cc: Frederic Weisbecker <fweisbec@...il.com>
> > Cc: Ingo Molnar <mingo@...hat.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.


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.

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