linux-kernel - Re: [PATCH] tracing/filters: allow event filters to be set only when not tracing

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Date:	Mon, 6 Apr 2009 15:52:55 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Tom Zanussi <tzanussi@...il.com>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing/filters: allow event filters to be set only when
 not tracing


On Mon, 6 Apr 2009, Frederic Weisbecker wrote:
> > > 
> > > > > > So assuming we can't use rcu for this, it would be nice to have a way to
> > > > > > 'pause' tracing so the current filter can be removed i.e. some version
> > > > > > of stop_trace()/start_trace() that make sure nothing is still executing
> > > > > > or can enter filter_match_preds() while the current call->preds is being
> > > > > > destroyed.  Seems like it would be straightforward to implement for the
> > > > > > event tracer, since each event maps to a tracepoint that could be
> > > > > > temporarily unregistered/reregistered, but maybe not so easy for the
> > > > > > ftrace tracers...
> > > > > 
> > > > > In principle, it would be possible to rework RCU so that instead of the
> > > > > whole idle loop being a quiescent state, there is a single quiescent state
> > > > > at one point in each idle loop.  The reason that I have been avoiding this
> > > > > is that there are a lot of idle loops out there, and it would be a bit
> > > > > annoying to (1) find them all and update them and (2) keep track of all of
> > > > > them to ensure that new ones cannot slip in without the quiescent state.
> > > > > 
> > > > > But it could be done if the need is there.  Simple enough change.
> > > > > The following patch shows the general approach, assuming that CPUs
> > > > > are never put to sleep without entering nohz mode.
> > > > > 
> > > > > Thoughts?
> > > > 
> > > > I think using synchronize_sched() should be good enough for what we need.
> > > 
> > > Again, as long as either (1) you are OK with synchronize_sched()
> > > ignoring preempt-disable sequences in the idle loop or (2) we rework RCU
> > > to add something like an rcu_idle() call in each idle loop.
> > 
> > 3) add "notrace" to the idle functions ;-)
> > 
> > But perhaps the rcu_idle might be the best idea.
> 
> 
> And tracing the idle time is also sometimes very useful :-)

Agreed. I guess choice 2 is the best answer.

-- Steve

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