linux-kernel - Re: [PATCH] tracing/ftrace: add missing wake-up on some callsites

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Date:	Mon, 23 Feb 2009 17:26:31 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [PATCH] tracing/ftrace: add missing wake-up on some callsites

On Mon, Feb 23, 2009 at 10:42:04AM -0500, Steven Rostedt wrote:
> 
> On Mon, 23 Feb 2009, Steven Rostedt wrote:
> 
> > 
> > 
> > On Sun, 22 Feb 2009, Frederic Weisbecker wrote:
> > 
> > > Impact: fix unwaken pipe
> > > 
> > > Now that we use a common wakeup infrastructure, we must append a wakeup
> > > on few callsites which lack it or tasks reading trace_pipe will not be
> > > awaken when events come on few tracers.
> > > 
> > > Cc: Steven Rostedt <rostedt@...dmis.org>
> > > Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
> > > Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> > > ---
> > >  kernel/trace/trace.c             |    2 ++
> > >  kernel/trace/trace_branch.c      |    2 ++
> > >  kernel/trace/trace_hw_branches.c |    2 ++
> > >  3 files changed, 6 insertions(+), 0 deletions(-)
> > > 
> > > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> > > index e1f3b99..7f450b6 100644
> > > --- a/kernel/trace/trace.c
> > > +++ b/kernel/trace/trace.c
> > > @@ -3055,6 +3055,8 @@ int trace_vprintk(unsigned long ip, int depth, const char *fmt, va_list args)
> > >   out:
> > >  	preempt_enable_notrace();
> > >  
> > > +	trace_wake_up();
> > > +
> > >  	return len;
> > >  }
> > >  EXPORT_SYMBOL_GPL(trace_vprintk);
> > > diff --git a/kernel/trace/trace_branch.c b/kernel/trace/trace_branch.c
> > > index c2e68d4..8c8f8c0 100644
> > > --- a/kernel/trace/trace_branch.c
> > > +++ b/kernel/trace/trace_branch.c
> > > @@ -78,6 +78,8 @@ probe_likely_condition(struct ftrace_branch_data *f, int val, int expect)
> > >   out:
> > >  	atomic_dec(&tr->data[cpu]->disabled);
> > >  	local_irq_restore(flags);
> > > +
> > > +	trace_wake_up();
> > >  }
> > >  
> > >  static inline
> > > diff --git a/kernel/trace/trace_hw_branches.c b/kernel/trace/trace_hw_branches.c
> > > index 3561aac..ddd87fd 100644
> > > --- a/kernel/trace/trace_hw_branches.c
> > > +++ b/kernel/trace/trace_hw_branches.c
> > > @@ -202,6 +202,8 @@ void trace_hw_branch(u64 from, u64 to)
> > >   out:
> > >  	atomic_dec(&tr->data[cpu]->disabled);
> > >  	local_irq_restore(irq1);
> > > +
> > > +	trace_wake_up();
> > >  }
> > >  
> > >  static void trace_bts_at(const struct bts_trace *trace, void *at)
> > 
> > Ah, we don't wake up purposely on these three places. ftrace_printk is 
> > meant to be called anywhere (including the scheduler). And the branch 
> > tracers are also allowed to be called anywhere (they usually are).
> > 
> > Calling "wake_up" from any of these can easily cause a dead lock with the 
> > run queue lock, because all three can be called from with in the 
> > scheduler.
> > 
> > Sorry, but I have to NACK this change.
> 
> 
> Perhaps we could add these callsites back, but we would need to update 
> trace_wake_up.
> 
> Have trace_wake_up set a flag instead, and add a tracepoint around the 
> scheduler (outside the grabbing of runqueue locks), that will have a 
> callback to the tracing code. That call back can perform the wakeups.
> 
> How does that sound?


That sounds good but only for these particular tracers I guess.

 
> -- Steve
> 

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