Re: [RFC PATCH 03/10] ftrace: Drop the ftrace_profile_enabledchecks in tracing hot path

[Mathieu Desnoyers]

* Steven Rostedt (rostedt@xxxxxxxxxxx) wrote:
> On Thu, 2010-01-21 at 21:28 -0500, Mathieu Desnoyers wrote:
> > * Steven Rostedt (rostedt@xxxxxxxxxxx) wrote:
> 
> > > Now for the reason I Cc'd Paul and Mathieu...
> > > 
> > > If we had a synchronize_sched() like function that would wait and return
> > > when all preempted tasks have been scheduled again and went to either
> > > userspace or called schedule directly, then we could actually do this.
> > > 
> > > After unregistering the function graph trace, you call this
> > > "synchronize_tasks()" and it will guarantee that all currently preempted
> > > tasks have either went to userspace or have called schedule() directly.
> > > Then it would be safe to remove this check.
> > 
> > OK, so basically you need to know when you reach a quiescent state, but
> > preemption is enabled and there is no RCU read lock taken around these
> > code paths, am I correct ?
> > 
> > With tracepoints, life is easy because I disable preemption around the
> > calls, so I can use synchronize_sched() to know when quiescent state is
> > reached.
> > 
> > I recommend looking at kernel/kprobes.c:check_safety(). It uses
> > thaw_processes() and synchronize_sched() for this purpose. Basically, it
> > rely on the "refrigeration" points to detect such quiescent state. This
> > trick should do the job for the function graph tracer too.
> > 
> > I'm adding Masami in CC. He is the one who implemented check_safety(),
> > and I remember discussing it with him in the past.
> 
> Hmm, interesting. Maybe something like that might work. But what if
> CONFIG_PREEMPT is enabled but CONFIG_FREEZER is not?

Then you may want to make the function tracer depend on CONFIG_FREEZER,
but maybe Masami has other ideas ?

Mathieu

> 
> -- Steve
> 
> 

-- 
Mathieu Desnoyers
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