linux-kernel - Re: [PATCH] tracing: Allow raw_syscall tracepoints to work from boot

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Date:	Fri, 16 Jan 2015 09:47:47 +1100
From:	Michael Ellerman <mpe@...erman.id.au>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...hat.com,
	paulmck@...ux.vnet.ibm.com, Andrew Morton <akpm@...l.org>,
	tglx@...utronix.de, mathieu.desnoyers@...icios.com,
	xiakaixu@...wei.com
Subject: Re: [PATCH] tracing: Allow raw_syscall tracepoints to work from boot

On Thu, 2015-01-15 at 08:58 -0500, Steven Rostedt wrote:
> On Thu, 15 Jan 2015 17:10:40 +1100
> Michael Ellerman <mpe@...erman.id.au> wrote:
> > 
> > I like my version better, but your call.
> 
> Of course you do :-)

You've got to admit mine is a lot neater looking :)

> I thought about it a bit, and both versions are really hacks. But in
> the end, I'd rather not touch the swapper task because that might give
> us some unwanted side effects.

Yeah that's true. I don't *think* there would be, but touching swapper is
certainly something one should do with caution.

> I don't really like my approach where I need to disable and re-enable
> all tracepoints. I was thinking of only enabling and disabling just the
> syscall ones, but I could imagine another tracepoint with a reg that
> could be affected by early boot as well, so I left it touching all
> events. My patch is fine for mainline, but I could make a patch for
> 3.20 that will only restart a tracepoint if it has its own reg/unreg
> functions and does not use the default ones.
> 
> Your patch fixes syscall events. I wanted something that will fix any
> event with its own special registration that might also use
> for_each_process_thread() or some other call that does not work before
> init is created.

Yep, that is definitely a benefit. I don't think there are heaps of folks using
tracepoints from boot, so it's possible something else was broken and we
haven't noticed yet.

cheers


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