linux-kernel - Re: [Patch] Tracing/ftrace: Adds a marker to allow user comments

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Message-ID: <20080907171143.3acdbedd@daedalus.pq.iki.fi>
Date:	Sun, 7 Sep 2008 17:11:43 +0300
From:	Pekka Paalanen <pq@....fi>
To:	"Steven Rostedt" <rostedt@...dmis.org>
Cc:	"Frédéric Weisbecker" <fweisbec@...il.com>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Linux Kernel" <linux-kernel@...r.kernel.org>
Subject: Re: [Patch] Tracing/ftrace: Adds a marker to allow user comments

Steven,

is there a logic behind trace_seq_print_cont() printing the terminating
newline only, when there actually is no TRACE_CONT entry?

Hmm, wait a minute, I don't understand how this thing works at all.

Let's take for instance print_lat_fmt() which is the first user of
trace_seq_print_cont(). Now, print_lat_fmt() does

	struct trace_entry *entry = iter->ent;

and uses 'entry'. Let's assume it is of type TRACE_PRINT. It does

	case TRACE_PRINT:
		seq_print_ip_sym(s, field->print.ip, sym_flags);
		trace_seq_printf(s, ": %s", field->print.buf);
		if (field->flags & TRACE_FLAG_CONT)
			trace_seq_print_cont(s, iter);
		break;

Ok, it prints the beginning of the message, and if there should be
continuation blocks, it calls trace_seq_print_cont(), defined as
follows:

static void
trace_seq_print_cont(struct trace_seq *s, struct trace_iterator *iter)
{
	struct trace_array *tr = iter->tr;
	struct trace_array_cpu *data = tr->data[iter->cpu];
	struct trace_entry *ent;

	ent = trace_entry_idx(tr, data, iter, iter->cpu);
	if (!ent || ent->type != TRACE_CONT) {
		trace_seq_putc(s, '\n');
		return;
	}

	do {
		trace_seq_printf(s, "%s", ent->cont.buf);
		__trace_iterator_increment(iter, iter->cpu);
		ent = trace_entry_idx(tr, data, iter, iter->cpu);
	} while (ent && ent->type == TRACE_CONT);
}

Here it uses trace_entry_idx() to get 'ent'. What's the difference to
iter->ent? I don't understand how trace_entry_idx() works, but looking
at how it is used, it must return the pointer to the *next* entry in
the ring buffer. So I don't understand the name of the function, and I
don't see a call to __trace_iterator_increment(), which is confusing.

If contrary to the assumption, 'ent' is not a continuation, it prints
the terminating newline. This is an exceptional case, as the original
entry was marked as having continuation entries.

The normal case then is to execute the do-while, until it hits a
non-continuation entry. Here it does *not* print the terminating newline.

Steven, could you explain what is going on here?

Thanks.

On Thu, 4 Sep 2008 20:30:58 +0300
Pekka Paalanen <pq@....fi> wrote:

> b) have a customisable trace_seq_print_cont()
> 
> I think I'd favour b). When I tried to use trace_seq_print_cont(),
> I found it difficult to control the printing of \n at the end of
> message. So I would implement my own routine to handle also what
> trace_seq_print_cont() does, filtering the text while copying it
> to struct trace_seq. I'm not sure where this function should live.
> It could be useful to all tracers, that want "one line per marker
> entry" style output, so it could live in trace.c.

-- 
Pekka Paalanen
http://www.iki.fi/pq/
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