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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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