Date: Fri, 3 Apr 2009 15:59:56 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Tom Zanussi <tzanussi@gmail.com>, "Paul E. McKenney" <paulmck@us.ibm.com>
Cc: linux-kernel <linux-kernel@vger.kernel.org>,
       Steven Rostedt <rostedt@goodmis.org>, fweisbec@gmail.com
Subject: Re: [PATCH] tracing/filters: allow event filters to be set only
	when not tracing
Message-ID: <20090403135956.GD8875@elte.hu>
References: <1238390546.6368.65.camel@bookworm> <20090401122408.GG12966@elte.hu> <1238653371.6655.48.camel@bookworm>
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* Tom Zanussi <tzanussi@gmail.com> wrote:

> On Wed, 2009-04-01 at 14:24 +0200, Ingo Molnar wrote:
> > * Tom Zanussi <tzanussi@gmail.com> wrote:
> > 
> > > This patch adds code allowing the event filter to be set only if 
> > > there's no active tracing going on.
> > 
> > > --- a/kernel/trace/trace_events.c
> > > +++ b/kernel/trace/trace_events.c
> > > @@ -498,6 +498,9 @@ event_filter_write(struct file *filp, const char __user *ubuf, size_t cnt,
> > >  	struct filter_pred *pred;
> > >  	int err;
> > >  
> > > +	if (tracing_is_enabled() && (!tracer_is_nop() || call->enabled))
> > > +		return -EBUSY;
> > 
> > hm, but it would be the normal use-case to set filters on the fly. 
> > To experiment around with them and shape them until the output is 
> > just right. Having to turn the tracer on/off during that seems quite 
> > counterproductive to that use-case.
> > 
> 
> I didn't see anything that could be used to temporarily disable 
> tracing (tracing_stop() and tracing_start() are 'quick' versions 
> that mostly just disable recording), so did it this way to avoid 
> adding any overhead to the filter-checking code.
> 
> But anyway, I'll post a new patch shortly that uses rcu and does 
> allow the filters to be set on the fly.

that's a very intelligent way to do it!

There's a theoretical problem though: what if we put a filtered 
tracepoint into the RCU code? Especially if that tracepoint is in 
the common function-tracer callback affecting all kernel functions. 
I've Cc:-ed Paul. I think the quiescent state logic should handle 
this just fine, but i'm not 100% sure.

	Ingo
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