linux-kernel - Re: ftrace not working?

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Date:	Tue, 24 Mar 2009 21:13:58 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Nikanth Karthikesan <knikanth@...e.de>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Steven Rostedt <srostedt@...hat.com>, rusty@...tcorp.com.au,
	Ingo Molnar <mingo@...e.hu>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-kernel@...r.kernel.org
Subject: Re: ftrace not working?

On Tue, Mar 24, 2009 at 10:53:35PM +0530, Nikanth Karthikesan wrote:
> On Monday 23 March 2009 20:02:26 Ingo Molnar wrote:
> > * KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:
> > > > On Monday 23 March 2009 14:05:33 Ingo Molnar wrote:
> > > > > one thing to note:
> > > > > | CPU0: Intel QEMU Virtual CPU version 0.9.1 stepping 03
> > > > > | Testing tracer nop: PASSED
> > > > >
> > > > > so it's a Qemu session, right?
> > > >
> > > > Yes. qemu-kvm.
> > >
> > > Ah, sorry. I don't know qemu and kvm at all ;)
> >
> > In terms of tracing it should just look like some weird old CPU with
> > few capabilities and long delays. Nothing in ftrace depends on CPU
> > capabilities (that wasnt present in pentia) so this is weird.
> > Perhaps some timestamping problem? Virtual machines tend to have
> > trouble with the TSC. Or code patching problems? But that does not
> > explain the lack of explicit scheduler events.
> >
> > Or perhaps the fact that qemu sessions tend to boot with barely any
> > RAM. Maybe the ringbuffer size is very low?
> >
> 
> Nope, it is not a qemu problem, but ftrace. Most probably, it shouldn't work on
> real hardware as well with my config. i.e., CONFIG_CPUMASK_OFFSTACK = y
> and NR_CPUS = 4096 via CONFIG_MAXSMP.
> 
> The tracing_buffer_mask was getting freed and the bitmask was all zeros. And
> for_each_tracing_cpu() never looped. Here is the patch that fixes the issue
> for me. Tested only on qemu-kvm. ;)
> 
> Steve, Can you merge this.
> 
> Thanks
> Nikanth
> 
> In tracer_alloc_buffers(), do not free tracing_cpumask and tracing_buffer_mask
> on success.
> 
> Signed-off-by: Nikanth Karthikesan <knikanth@...e.de>
> 
> ---
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 17bb88d..98f609d 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -3887,6 +3887,7 @@ __init static int tracer_alloc_buffers(void)
>  
>  	register_die_notifier(&trace_die_notifier);
>  	ret = 0;
> +	goto out;


Ah I see.
I've sent a fix for that already, it's on latest -tip
http://git.kernel.org/?p=linux/kernel/git/tip/linux-2.6-tip.git;a=commit;h=2fc1dfbe17e7705c55b7a99da995fa565e26f151


  
>  out_free_cpumask:
>  	free_cpumask_var(tracing_cpumask);
> 
> 

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