linux-kernel - Re: [PATCH 1/2] tracing: Add $cpu and $current probe-vars

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Message-ID: <20131127170550.GB26138@redhat.com>
Date:	Wed, 27 Nov 2013 18:05:50 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Namhyung Kim <namhyung.kim@....com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>, Jiri Olsa <jolsa@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] tracing: Add $cpu and $current probe-vars

On 11/27, Masami Hiramatsu wrote:
>
> (2013/11/27 2:23), Oleg Nesterov wrote:
> > On 11/26, Masami Hiramatsu wrote:
> >>
> >> (2013/11/26 4:29), Oleg Nesterov wrote:
> >>> +#define PSEUDO_REG_OFFSET	4096	/* arbitrary value > MAX_REG_OFFSET */
> >>> +
> >>> +static unsigned long probe_get_register(struct pt_regs *regs, unsigned long offset)
> >>> +{
> >>> +	if (offset < PSEUDO_REG_OFFSET)
> >>> +		return regs_get_register(regs, offset);
> >>> +
> >>> +	return pseudo_reg_table[offset - PSEUDO_REG_OFFSET].fetch();
> >>> +}
> >>
> >>
> >> Hmm, I don't like this, since fetch_reg is the most frequently
> >> used fetch method, and it actually uses the offset in different
> >> way.
> >
> > Sure, this overloads the usage of FETCH_MTD_reg/offset.
> >
> > And btw, yes, the naming is ugly (I mean pseudo_*). But why this
> > is bad? This is cheap and simple.
>
> I think it's not simple. The code looks short, but not well
> self-described. It is "hidden" in the structure, and double-meaning.

Yes, I agree. And the ugly naming doesn't make it more clear/clean.

I'll try to cleanup this somehow and resend. Perhaps I should start
with 2/2 which generalizes FETCH_MTD_reg and kills FETCH_MTD_retval.
Or at least generalizes FETCH_MTD_retval.

> >> Why don't you make another fetch function for vars?
> >
> > This is what I tried to avoid ;) I do not want to add another
> > FETCH_MTD_. To me, this looks like unnecessary complication and
> > bloat (but see below).
>
> I see, perhaps, it is a time to introduce independent fetch
> method implementation. Current one is not sophisticated
> nor generic

Yes, yes, I agree. But until then I do not want to complicate/blow
this code to implement $current.

Thanks.

Oleg.

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