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    SubjectRe: [PATCH] new irq tracer
    * Jason Baron (jbaron@redhat.com) wrote:
    > On Wed, Feb 25, 2009 at 11:48:28AM -0500, Masami Hiramatsu wrote:
    > > KOSAKI Motohiro wrote:
    > > >> /**
    > > >> * handle_IRQ_event - irq action chain handler
    > > >> * @irq: the interrupt number
    > > >> @@ -354,7 +358,9 @@ irqreturn_t handle_IRQ_event(unsigned int irq, struct irqaction *action)
    > > >> local_irq_enable_in_hardirq();
    > > >>
    > > >> do {
    > > >> + trace_irq_entry(irq);
    > > >> ret = action->handler(irq, action->dev_id);
    > > >> + trace_irq_exit(irq, ret);
    > > >> if (ret == IRQ_HANDLED)
    > > >> status |= action->flags;
    > > >> retval |= ret;
    > > >
    > > > Nobdy want unnecessary redundant tracepoint.
    > > > Please discuss with mathieu, and merge his tracepoint.
    > >
    > > Hmm, from the viewpoint of trouble shooting, the place of LTTng's tracepoint
    > > is enough. However, from the same viewpoint, it should pass irq-number
    > > to irq-exit event too, because we may lost some previous events by buffer-overflow
    > > etc.
    > >
    > > trace_irq_entry(irq, NULL);
    > > ret = _handle_IRQ_event(irq, action);
    > > trace_irq_exit(irq, ret);
    > > ^^^^
    > >
    >
    > the lttng tracepoints wrap the calls to _handle_IRQ_event in 3
    > different places. So the above suggested irq tracepoint provides the
    > same information with 4 less tracepoints in the code. So I believe its
    > simpler - plus we can understand which action handlers are handling the
    > interrupt.
    >

    The main thing I dislike about only tracing action->handler() calls is
    that you are not tracing an IRQ per se, but rather the invocation of a
    given handler within the interrupt. For instance, it would be difficult
    to calculate the maximum interrupt latency for a given interrupt line,
    because you don't have the "real" irq entry/exit events, just the
    individual handler() calls.

    But I agree that knowing which handler is called is important.

    How about this compromise :

    trace_irq_entry(irq, action)
    _handle_IRQ_event()
    for each action {
    trace_irq_handler(action, ret);
    ret = action->handler(irq, action->dev_id);
    ...
    }
    trace_irq_exit(action_ret);

    Would that give you the information you need ?

    Here trace_irq_handler would be passed the _current_ action invoked and
    the _previous_ action return value. Note that we should initialize
    irqreturn_t ret to some initial value if we do this. That should keep
    the tracing overhead minimal.

    Mathieu

    > thanks,
    >
    > -Jason
    >

    --
    Mathieu Desnoyers
    OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68


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