linux-kernel - Re: [RFC PATCH] seq_file: Use seq_puts when seq

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Date:	Sat, 16 Mar 2013 10:51:18 -0700
From:	Joe Perches <joe@...ches.com>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] seq_file: Use seq_puts when seq_printf has only a
 format with no args

On Sat, 2013-03-16 at 09:43 -0600, Bjorn Helgaas wrote:
> On Sat, Mar 16, 2013 at 7:50 AM, Joe Perches <joe@...ches.com> wrote:
> > Instead of converting the 800 or so uses of seq_printf with
> > a constant format (without a % substitution) to seq_puts,
> > maybe there's another way to slightly speed up these outputs.
> >
> > Taking a similar approach to commit abd84d60eb
> > ("tracing: Optimize trace_printk() with one arg to use trace_puts()")
> > use the preprocessor to convert seq_printf(seq, "string constant")
> > to seq_puts(seq, "string constant")
> >
> > By stringifying __VA_ARGS__, we can, at compile time, determine
> > the number of args that are being passed to seq_printf() and
> > call seq_puts or seq_printf appropriately.
> >
> > The actual function definition for seq_printf must now
> > be enclosed in parenthesis to avoid further macro expansion.
> 
> This is certainly a neat trick.
> 
> But I don't really like the fact that it complicates things for every
> future code reader, especially when a trivial change in the caller
> would accomplish the same thing.  Do you have any idea how much
> performance we would gain in exchange for the complication?

Nope.  I believe it's trivial in any case.
I just saw Steven's trace hack and thought of seq_printk.

Is there a real performance sensitive seq_printf anywhere?

It's trivial to replace seq_printf("constant") with
seq_puts but there are over a thousand of them.

It may be better to just leave everything as-is.

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