linux-kernel - Re: [PATCH 4/4] tracing: Add __print_ns_to_secs() and __print_ns_without

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Date:	Tue, 23 Feb 2016 15:01:43 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Juri Lelli <juri.lelli@...il.com>,
	Clark Williams <williams@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 4/4] tracing: Add __print_ns_to_secs() and
 __print_ns_without_secs() helpers

On Tue, Feb 23, 2016 at 08:17:57AM -0500, Steven Rostedt wrote:
> On Tue, 23 Feb 2016 13:49:15 +0100
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Mon, Feb 22, 2016 at 04:26:53PM -0500, Steven Rostedt wrote:
> > > From: "Steven Rostedt (Red Hat)" <rostedt@...dmis.org>
> > > 
> > > To have nanosecond output displayed in a more human readable format, its
> > > nicer to convert it to a seconds format (XXX.YYYYYYYYY). The problem is that
> > > to do so, the numbers must be divided by NSEC_PER_SEC, and moded too. But as
> > > these numbers are 64 bit, this can not be done simply with '/' and '%'
> > > operators, but must use do_div() instead.  
> > 
> > Would not div_[us]64_rem() make more sense? It would typically result in
> > just the one division, instead of two.
> 
> The problem is, how do you do that in a printf() statement?
> 
> We have "%llu.%09ul" which is two arguments in the printf(). And the
> values we are processing can't be modified. Which is why the macro uses
> ({ }) and creates a temp variable.

Hurm,. yes that's not something easily done. We need a temporary limited
in scope to the printf statement, and C doesn't really do that.