linux-kernel - Re: [PATCH v3] ftrace: add a tracepoint for __raise_softirq

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Date:	Mon, 11 May 2009 11:13:53 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, fweisbec@...il.com,
	zhaolei@...fujitsu.com, laijs@...fujitsu.com,
	Li Zefan <lizf@...fujitsu.com>
Subject: Re: [PATCH v3] ftrace: add a tracepoint for
	__raise_softirq_irqoff()

* Steven Rostedt (rostedt@...dmis.org) wrote:
> 
> 
> 
> On Mon, 11 May 2009, Mathieu Desnoyers wrote:
> 
> > * Steven Rostedt (rostedt@...dmis.org) wrote:
> > > 
> > > On Mon, 11 May 2009, Mathieu Desnoyers wrote:
> > > > 
> > > > Yes, we should try to fix TRACE_EVENT, but we should fix it _before_ we
> > > > start using it widely. Circular header dependencies is a real problem
> > > > with TRACE_EVENT right now.
> > > > 
> > > > Until we fix this, I will be tempted to stay with a known-good solution,
> > > > which is DECLARE/DEFINE_TRACE.
> > > 
> > > The majority of tracepoints happen is C files. Those few cases where they 
> > > are used in headers is where the issues arise.
> > > 
> > > But...
> > > 
> > > I did not want to uglify all trace event headers with:
> > > 
> > > #ifdef CREATE_FOO_TRACE_POINTS
> > > #undef CREATE_FOO_TRACE_POINTS
> > > #include <trace/define_trace.h>
> > > #endif
> > > 
> > > We would only need to do that for those trace points that need to be 
> > > included in header files. Then the declaration C file would need to define 
> > > both CREATE_FOO_TRACE_POINTS and CREATE_TRACE_POINTS
> > > 
> > > #define CREATE_FOO_TRACE_POINTS
> > > #define CRATE_TRACE_POINTS
> > > #include <trace/events/foo.h>
> > > 
> > > 
> > > But this is pretty trivial to solve, and I do not consider it a show 
> > > stopper or a major header dependency problem.
> > > 
> > > -- Steve
> > > 
> > 
> > Hrm, is there any way to solve it elegantly ?
> > 
> > What we really need is to see the cases where TRACE_EVENT() is used as a
> > declaration vs the case where it expands
> > TP_STRUCT__entry/TP_fast_assign/TP_printk as having different
> > dependencies. The problem comes when we bring the include dependencies
> > of the TP_fast_assign part into the tracepoint header and it becomes
> > a dependency of the TRACE_EVENT() declaration-only part.
> > 
> > Can we do the following ?
> > 
> > All tracepoint headers could surround the include dependencies by :
> > 
> > #ifdef BUILD_EVENTS
> > #include <veryannoyingheaderdependency.h>
> > #endif
> > 
> > And then we follow this by the TRACE_EVENT() declarations.
> > 
> > BUILD_EVENTS would only be defined in kernel/trace/events.c.
> 
> That file no longer exists. All the events are created with the 
> define_trace.h or ftrace.h file.
> 
> We could do something like your
> 
> #ifdef BUILD_EVENTS /* or better name BUILDING_EVENTS ?? */
> #include <allmyannoyingheaders.h>
> [...]
> #endif
> 
> And then have the define_trace.h file add define it. But this again adds 
> more annoying CPP commands to all trace_event headers.
> 

Yes, but this would solve most of the include dependency problems at
once. We would only have dependency on preempt.h left, which alone is
easier to deal with. And if headers don't need to include any annoying
headers, they don't need to use the ifdef BUILDING_EVENTS. But my point
is that when it's needed (and we already see two cases where it's
needed, with pvops instrumentation and softirq instrumentation, and I
guess we'll see much more), it's good to have this infrastructure in
place, so people don't end up trying to do hacks to the kernel code
changing inlines for function calls to try to deal with the circular
include issue.

I think it's

a) needed
b) it does not hurt anyone who does not need it.

So I would definitely recommend adding such define.

Mathieu

> -- Steve
> 
> > 
> > I think it should work, but it looks a bit too simple, so I may have
> > missed something... ?
> > 
> > Mathieu
> > 
> > 
> > -- 
> > Mathieu Desnoyers
> > OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
> > 

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