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

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Date:	Wed, 25 Feb 2009 11:58:49 -0500
From:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
To:	Masami Hiramatsu <mhiramat@...hat.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Jason Baron <jbaron@...hat.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

* Masami Hiramatsu (mhiramat@...hat.com) 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);
>                         ^^^^
> 

I seriously doubt we should consider a trace with missing events as
"reliable". If your only argument is that when the buffers are not large
enough we could lose events, then I think we should just hint people at
doing the right thing, which is to tweak the tracer parameters (e.g.
larger buffers) so they stop losing events.

A trace with events lost is really a scenario close to a corrupted
trace because we don't know which event has been lost, nor where. I
don't think we should increase the event size to support that kind of
broken scenario.

Mathieu

> Thank you,
> 
> -- 
> Masami Hiramatsu
> 
> Software Engineer
> Hitachi Computer Products (America) Inc.
> Software Solutions Division
> 
> e-mail: mhiramat@...hat.com
> 

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