netdev - Re: [RFC PATCH v3 1/5] irq: add tracepoint to softirq

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Message-Id: <20100721215836.86F9.A69D9226@jp.fujitsu.com>
Date:	Wed, 21 Jul 2010 22:01:34 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Neil Horman <nhorman@...driver.com>
Cc:	kosaki.motohiro@...fujitsu.com,
	Koki Sanagi <sanagi.koki@...fujitsu.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	davem@...emloft.net, kaneshige.kenji@...fujitsu.com,
	izumi.taku@...fujitsu.com, laijs@...fujitsu.com,
	scott.a.mcmillan@...el.com, rostedt@...dmis.org,
	eric.dumazet@...il.com, fweisbec@...il.com,
	mathieu.desnoyers@...ymtl.ca
Subject: Re: [RFC PATCH v3 1/5] irq: add tracepoint to softirq_raise

> > >>  #endif /*  _TRACE_IRQ_H */
> > >> diff --git a/kernel/softirq.c b/kernel/softirq.c
> > >> index 825e112..6790599 100644
> > >> --- a/kernel/softirq.c
> > >> +++ b/kernel/softirq.c
> > >> @@ -215,9 +215,9 @@ restart:
> > >>  			int prev_count = preempt_count();
> > >>  			kstat_incr_softirqs_this_cpu(h - softirq_vec);
> > >>  
> > >> -			trace_softirq_entry(h, softirq_vec);
> > >> +			trace_softirq_entry(h - softirq_vec);
> > >>  			h->action(h);
> > >> -			trace_softirq_exit(h, softirq_vec);
> > >> +			trace_softirq_exit(h - softirq_vec);
> > > 
> > > You're loosing information here by reducing the numbers of parameters in this
> > > tracepoint.  How many other tracepoint scripts rely on having both pointers
> > > handy?  Why not just do the pointer math inside your tracehook instead?
> > 
> > In __raise_softirq_irqoff macro there is no method to refer softirq_vec, so it
> > can't use softirq DECLARE_EVENT_CLASS as is.
> > Currently,  there is no script using softirq_entry or softirq_exit.
> > 
> That shouldn't matter, just pass in NULL for softirq_vec in
> __raise_softirq_irqoff as the second argument to the trace function.  You may
> need to fix up the class definition so that the assignment or printk doesn't try
> to dereference that pointer when its NULL, but thats easy enough, and it avoids
> breaking any other perf scripts floating out there.

please see 5 lines above. we already have 'h - softirq_vec' calculation in
this function. so, Sanagi-san's change don't makes any overhead.

So, if the overhead is zero, I'd prefer simplest tracepoint definition :)



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