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

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Date:	Wed, 25 Feb 2009 13:05:35 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <compudj@...stal.dyndns.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



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.

 # 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.
> 

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