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SubjectRe: ftrace behaviour (was: [PATCH] ftrace: introduce tracing_reset_online_cpus() helper)
On Fri, 19 Dec 2008 19:29:30 -0500 (EST)
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Sat, 20 Dec 2008, Ingo Molnar wrote:
> >
> > The current "only allow resizing while tracing is disabled" is an
> > unintuitive and counterproductive restriction - a borderline bug in fact.
>
> Having to disable tracing to resize is much better than having to reboot.
>
> I'm not defending that this is the way to go. I never planned on keeping
> this the defacto standard. I've been setting up infrastructure to allow
> for a seamless resize. If you think we should reset the buffers on resize,
> then that would be a great way for the user to know why they are missing
> **all** their data.

I thought this was just about not having to do

$ echo 0 > tracing_enabled
$ echo 28764243 > buffer_size
$ echo 1 > tracing_enabled

and instead just do

$ echo 28764243 > buffer_size

which would do exactly the same, except being easier for the user.
Personally I've never dreamed of any kind of resize-in-flight.

> What I'm trying to say is that the more we allow during resize, the more
> likely there will be a critical bug that might lock up the system. The
> whole point about writing the ring buffer was to do it incrementally.
> Start out with being straight forward and simple, then we could add
> features as we understand things better.
>
> IOW, I totally agree that we should make it as intuitive as possible. But
> this needs to be done step by step. I wrote 10 different versions of the
> ring buffer in a week. Most of them were complete rewrites. I want this to
> be as robust and powerful as you do, but I also want to be cautious about
> doing things that might crash the system.
>
> Ftrace already had the infrastructure in place to protect against resize.
> I just used it to give me time to do other things with the ring buffer.
> I always planned on having the resize protection back inside the ring
> buffer code when I got around to it.
>
> -- Steve

--
Pekka Paalanen
http://www.iki.fi/pq/


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