linux-kernel - Re: [PATCH] new irq tracer

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Date:	Wed, 25 Feb 2009 12:34:12 -0500
From:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
To:	Jason Baron <jbaron@...hat.com>
Cc:	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,
	rostedt@...dmis.org, linux-kernel@...r.kernel.org,
	acme@...stprotocols.net, fweisbec@...il.com
Subject: Re: [PATCH] new irq tracer

* 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.

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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