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    SubjectRe: [PATCH] new irq tracer
    On Thu, Feb 26, 2009 at 10:11:05AM -0500, Jason Baron wrote:
    > On Wed, Feb 25, 2009 at 12:34:12PM -0500, Mathieu Desnoyers wrote:
    > > * 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.
    > >
    >
    > maybe...although that would require re-arranging the 'while' loop in
    > 'handle_IRQ_event' from a do..while loop to a 'while' loop, which will
    > require an extra branch check, and then we still have to record the last 'ret'
    > value. I'm not that keen on re-arranging this for trace data...
    >
    > Using Steve's new 'DEFINE_TRACE_FMT', I can get function graph trace
    > as follows using the original two tracepoints (patch below):
    >
    > 3) | handle_IRQ_event() {
    > 3) | /* (irq_handler_entry) irq=28 handler=eth0 */
    > 3) | e1000_intr_msi() {
    > 3) 2.460 us | __napi_schedule();
    > 3) 9.416 us | }
    > 3) | /* (irq_handler_exit) irq=28 handler=eth0 return=handled */
    > 3) + 22.935 us | }
    >
    > thanks,
    >
    > -Jason
    >


    I'm impressed by this new TRACE_FMT system.
    It means that I will just need to toggle a value on a /debug/trace/events/irq_stuff/enable
    to have the useful informations as comments inside a trace, or in a whole dedicated traces.

    I've played with it a part of the night to test the bprintk patch, this is awesome!



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