Re: [Qemu-devel] [PATCH 2/5] trace-cmd: Use tracing directory to count CPUs

[Steven Rostedt]

On Thu, 2012-08-23 at 12:00 +0900, Masami Hiramatsu wrote:
> (2012/08/23 11:01), Masami Hiramatsu wrote:
> > (2012/08/22 22:41), Steven Rostedt wrote:
> >> On Wed, 2012-08-22 at 17:43 +0900, Yoshihiro YUNOMAE wrote:
> >>> From: Masami Hiramatsu <address@hidden>
> >>>
> >>> Count debugfs/tracing/per_cpu/cpu* to determine the
> >>> number of CPUs.
> >>
> >> I'm curious, do you find that sysconf doesn't return the # of CPUs the
> >> system has?
> > 
> > No, sysconf returns the number of hosts CPUs, not guests.
> > 
> >> I've had boxes where the per_cpu/cpu* had more cpus than the
> >> box actually holds. But this was a bug in the kernel, not the tool. This
> >> change log needs to have rational instead of just explaining what the
> >> patch does.
> > 
> > Ah, I see. Hmm, then this should be enabled by a command line
> > option or an environment variable.
> 
> Oops, I misunderstood. I'll add more comment for why this
> should be tried instead of sysconf.

And now that I understand why you are doing this, why not only do this
if the TRACE_AGENT or DEBUG_TRACING_DIR is defined. That is, if we are
doing it against a bare metal system, then sysconf should suffice, but
if we are tracing against a guest, then it should use the tracing
directory to determine the buffers.

We could add options to override this, but I would think the default
should just Do The Right Thing(tm).

-- Steve