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SubjectRe: [PATCH] tracing/filters: allow event filters to be set only when not tracing

* 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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