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

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Date:	Thu, 23 Aug 2012 05:08:06 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@...achi.com>,
	Amit Shah <amit.shah@...hat.com>,
	Anthony Liguori <anthony@...emonkey.ws>,
	Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...64.org>,
	"Franch Ch. Eigler" <fche@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Ingo Molnar <mingo@...hat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, qemu-devel@...gnu.org,
	yrl.pp-manager.tt@...achi.com
Subject: Re: [PATCH 2/5] trace-cmd: Use tracing directory to count CPUs

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 <masami.hiramatsu.pt@...achi.com>
> >>>
> >>> 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


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