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Date: Wed, 25 Feb 2009 17:12:10 -0500
From: Mathieu Desnoyers <compudj@...stal.dyndns.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Jason Baron <jbaron@...hat.com>,
Masami Hiramatsu <mhiramat@...hat.com>,
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
Peter Zijlstra <peterz@...radead.org>,
"Frank Ch. Eigler" <fche@...hat.com>, mingo@...e.hu,
linux-kernel@...r.kernel.org, acme@...stprotocols.net,
fweisbec@...il.com
Subject: Re: [PATCH] new irq tracer
* Steven Rostedt (rostedt@...dmis.org) wrote:
>
>
> On Wed, 25 Feb 2009, Mathieu Desnoyers wrote:
>
> > * Jason Baron (jbaron@...hat.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.
>
> Then use the function_graph tracer.
>
Sadly, the function tracer cannot be enabled on production systems.
Therefore it's not a usable solution in the context where I need this.
Mathieu
> # echo smp_apic_timer_interrupt > /debug/tracing/set_ftrace_filter
> # echo __irqentry_text_start >> /debug/tracing/set_ftrace_filter
> # echo function_graph > /debug/tracing/current_tracer
> # cat /debug/tracing/trace
>
> # tracer: function_graph
> #
> # CPU DURATION FUNCTION CALLS
> # | | | | | | |
> ------------------------------------------
> 1) kblockd-4012 => <idle>-0
> ------------------------------------------
>
> 1) ==========> |
> 1) + 35.783 us | smp_apic_timer_interrupt();
> ------------------------------------------
> 2) kstop/2-4013 => <idle>-0
> ------------------------------------------
>
> 2) ==========> |
> 2) + 13.819 us | smp_apic_timer_interrupt();
> ------------------------------------------
> 3) kstop/3-4014 => <idle>-0
> ------------------------------------------
>
> 3) ==========> |
> 3) + 24.919 us | __irqentry_text_start();
> ------------------------------------------
> 0) kstop/0-4011 => <idle>-0
> ------------------------------------------
>
> 0) ==========> |
> 0) + 24.444 us | smp_apic_timer_interrupt();
> ------------------------------------------
> 1) <idle>-0 => bash-3899
> ------------------------------------------
>
> 1) ==========> |
> 1) + 13.642 us | smp_apic_timer_interrupt();
> 2) ==========> |
> 2) + 15.014 us | smp_apic_timer_interrupt();
> 3) ==========> |
> 3) + 31.004 us | smp_apic_timer_interrupt();
> ------------------------------------------
> 0) <idle>-0 => sshd-3892
> ------------------------------------------
>
> 0) ==========> |
> 0) + 15.655 us | __irqentry_text_start();
> 0) ==========> |
> 0) + 37.722 us | __irqentry_text_start();
> ------------------------------------------
> 0) sshd-3892 => <idle>-0
> ------------------------------------------
>
> 0) ==========> |
> 0) + 26.139 us | __irqentry_text_start();
> 3) ==========> |
> 3) + 27.240 us | __irqentry_text_start();
> 3) ==========> |
> 3) + 21.987 us | __irqentry_text_start();
>
>
> And there you have your latencies ;-)
>
> With Jasons added trace points, we could add the to the event tracer
> and those would show up as comments here.
>
> -- Steve
>
>
>
> >
> > 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 Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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