linux-kernel - Re: [Patch 11/12] ftrace plugin for kernel symbol tracing usingHWBreakpoint interfaces

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Date:	Wed, 13 May 2009 01:32:54 +0530
From:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Benjamin Herrenschmidt <benh@....ibm.com>,
	maneesh@...ux.vnet.ibm.com, Roland McGrath <roland@...hat.com>,
	Masami Hiramatsu <mhiramat@...hat.com>
Subject: Re: [Patch 11/12] ftrace plugin for kernel symbol tracing
	usingHWBreakpoint interfaces - v4

On Tue, May 12, 2009 at 05:15:05PM +0200, Frederic Weisbecker wrote:
> On Tue, May 12, 2009 at 07:49:44PM +0530, K.Prasad wrote:
> > On Tue, May 12, 2009 at 12:14:29AM +0200, Frederic Weisbecker wrote:
> > > On Mon, May 11, 2009 at 05:25:02PM +0530, K.Prasad wrote:
> > > 
> > > > +static int ksym_trace_init(struct trace_array *tr)
> > > > +{
> > > > +	int cpu, ret = 0;
> > > > +
> > > > +	for_each_online_cpu(cpu)
> > > > +		tracing_reset(tr, cpu);
> > > > +
> > > > +	ksym_tracing_enabled = 1;
> > > > +	ksym_trace_array = tr;
> > > > +
> > > > +#ifdef CONFIG_FTRACE_SELFTEST
> > > > +	/* Check if we are re-entering self-test code during initialisation */
> > > > +	if (ksym_selftest_dummy)
> > > > +		goto ret_path;
> > > > +
> > > > +	ksym_selftest_dummy = 0;
> > > > +
> > > > +	/* Register the read-write tracing request */
> > > > +	ret = process_new_ksym_entry(KSYM_SELFTEST_ENTRY, HW_BREAKPOINT_RW,
> > > > +					(unsigned long)(&ksym_selftest_dummy));
> > > > +
> > > > +	if (ret < 0) {
> > > > +		printk(KERN_CONT "ksym_trace read-write startup test failed\n");
> > > > +		goto ret_path;
> > > > +	}
> > > > +	/* Perform a read and a write operation over the dummy variable to
> > > > +	 * trigger the tracer
> > > > +	 */
> > > > +	if (ksym_selftest_dummy == 0)
> > > > +		ksym_selftest_dummy++;
> > > > +ret_path:
> > > > +#endif /* CONFIG_FTRACE_SELFTEST */
> > > 
> > > 
> > > It means that each time your tracer is selected, it will perform a selftest.
> > > I think we only need this selftest once during the boot.
> > > I would rather see that in the real selftest callback (trace_selftest_startup_kysm).
> > >
> > 
> > > > +   if (ksym_selftest_dummy)
> > > > +           goto ret_path;
> > 
> > The above check will help prevent a re-run of the test everytime init is
> > executed.
> > 
> > A part of the selftest was kept in trace_ksym.c (and hence in
> > ksym_trace_init()) in order to use functions local to this file, such as
> > process_new_ksym_entry().
> 
> 
> 
> Ok.
> May be you could just compute your selftests on the breakpoint themselves,
> not on the entries in the ring buffer.
> 
> That's what do other tracers because it's usually convenient, but the main
> focuse is the event itself, because the ring buffer is already tested.
> The only thing you need to test is the hardware breakpoint triggering, then
> you wouldn't need anymore the trace_selftest.c helpers and you could
> implement your selftest directly in your tracer file.
> 
> I remember such thing has been discussed recently but I'm not sure
> whether it concerned the kprobe tracer or yours.
>

I agree that the ring buffer infrastructure need not be tested itself,
but to just check the trigger of HW Breakpoint without populating the
ring buffer will need a new breakpoint handler defined exclusively for
startup self-test, or additional checks in ksym_hbp_handler() that
executes everytime for normal exceptions too.

I will move all code from ksym_trace_init() to
trace_selftest_startup_kysm() though.

Thanks,
K.Prasad

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