linux-kernel - Re: [GIT PULL] tracing: make signal tracepoints more useful

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Date:	Mon, 16 Jan 2012 13:53:30 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Seiji Aguchi <saguchi@...hat.com>,
	linux-kernel@...r.kernel.org,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Subject: Re: [GIT PULL] tracing: make signal tracepoints more useful


* Steven Rostedt <rostedt@...dmis.org> wrote:

> On Mon, 2012-01-16 at 08:45 +0100, Ingo Molnar wrote:
> 
> > Looks good to me at a first (quick) sight, except this bit 
> > which changes the ABI:
> > 
> > > > 	-       TP_printk("sig=%d errno=%d code=%d comm=%s pid=%d",
> > > > 	+       TP_printk("sig=%d errno=%d code=%d comm=%s pid=%d grp=%d res=%d",
> > 
> > That's not how we change tracepoints generally - we add a new 
> > one and eventually phase out the old one. Which apps/tools rely 
> > on the old tracepoint? If it's exactly zero apps then we might 
> > be able to change it, but this needs to be investigated.
> 
> But this tracepoint wasn't changed, it was added on to. 
> There's a difference. Any tool that uses this (including 
> something like powertop) should be able to handle it. [...]

That's mostly true in theory - the question is, is it true in 
practice?

Say if an app relies on the smaller data structure, it sure 
might get surprised by the kernel writing a wider record ...

Thanks,

	Ingo
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