linux-kernel - Re: [Patch 11/11] ftrace plugin for kernel symbol tracing using HW Breakpoint interfaces

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Date:	Tue, 24 Mar 2009 23:03:18 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
cc:	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Benjamin Herrenschmidt <benh@....ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	maneesh@...ux.vnet.ibm.com, Roland McGrath <roland@...hat.com>
Subject: Re: [Patch 11/11] ftrace plugin for kernel symbol tracing using HW
 Breakpoint interfaces - v2


Hi Prasad,

On Tue, 24 Mar 2009, K.Prasad wrote:

> +
> +static void ksym_trace_print_header(struct seq_file *m)
> +{
> +
> +	seq_puts(m,
> +		 "#       TASK-PID      CPU#      Symbol         Type    "
> +		 "Function         \n");
> +	seq_puts(m,
> +		 "#          |           |          |              |         "
> +		 "|            \n");
> +}
> +
> +static enum print_line_t ksym_trace_output(struct trace_iterator *iter)
> +{
> +	struct trace_entry *entry = iter->ent;
> +	struct trace_seq *s = &iter->seq;
> +	struct trace_ksym *field;
> +	char str[KSYM_SYMBOL_LEN];
> +	int ret;
> +
> +	trace_assign_type(field, entry);

Looking at this, you do not test the iter->ent->type. If events or
trace_printk is running, this will fail your tracer.

You want to test for your types first, because all trace types will be 
called for "print_line" function below.

Thus you want something like

	if (entry->type != TRACE_KSYM)
		return TRACE_TYPE_UNHANDLED;

Then the tracer infrastructure could print out the trace event or 
trace_printk.

If you only want to print your events and ignore all others. Return
TRACE_TYPE_HANDLED.

Note, the if needs to be before the trace_assign_type.

-- Steve

> +
> +	ret = trace_seq_printf(s, "%-15s %-5d %-3d %-20s ", field->p_name,
> +				entry->pid, iter->cpu, field->ksym_name);
> +	if (!ret)
> +		return TRACE_TYPE_PARTIAL_LINE;
> +
> +	switch (field->ksym_hbp->info.type) {
> +	case HW_BREAKPOINT_WRITE:
> +		ret = trace_seq_printf(s, " W  ");
> +		break;
> +	case HW_BREAKPOINT_RW:
> +		ret = trace_seq_printf(s, " RW ");
> +		break;
> +	default:
> +		return TRACE_TYPE_PARTIAL_LINE;
> +	}
> +
> +	if (!ret)
> +		return TRACE_TYPE_PARTIAL_LINE;
> +
> +	sprint_symbol(str, field->ip);
> +	ret = trace_seq_printf(s, "%-20s\n", str);
> +	if (!ret)
> +		return TRACE_TYPE_PARTIAL_LINE;
> +
> +	return TRACE_TYPE_HANDLED;
> +}
> +
> +struct tracer ksym_tracer __read_mostly =
> +{
> +	.name		= "ksym_tracer",
> +	.init		= ksym_trace_init,
> +	.reset		= ksym_trace_reset,
> +#ifdef CONFIG_FTRACE_SELFTEST
> +	.selftest	= trace_selftest_startup_ksym,
> +#endif
> +	.print_header   = ksym_trace_print_header,
> +	.print_line	= ksym_trace_output
> +};
> +
--
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