linux-kernel - Re: [PATCH resend4 2/3] itimers: fix periodic tics precision

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Message-ID: <20090522163438.71f895d1@dhcp-lab-109.englab.brq.redhat.com>
Date:	Fri, 22 May 2009 16:34:38 +0200
From:	Stanislaw Gruszka <sgruszka@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH resend4 2/3] itimers: fix periodic tics precision

On Fri, 22 May 2009 16:27:40 +0200 (CEST)
Thomas Gleixner <tglx@...utronix.de> wrote:

> On Fri, 22 May 2009, Stanislaw Gruszka wrote:
> >  
> > +#define CPUTIME_SUB_NS(ct, real_ns) ({ 		\
> > +	struct timespec ts;			\
> > +	s64 cpu_ns;				\
> > +	cputime_to_timespec(ct, &ts); 		\
> > +	cpu_ns = timespec_to_ns(&ts);		\
> > +	cpu_ns - real_ns;	 		\
> > +})
> > +
> 
>   Please make this an inline function. Also this should have a sanity
>   check for values < 0, which might happen due to rounding errors. In
>   that case you set it simply to 0.

Ok.

 
> >  	if (cputime_ge(cur_time, it->expires)) {
> > -		it->expires = it->incr;
> > -		if (!cputime_eq(it->expires, cputime_zero))
> > -			it->expires = cputime_add(it->expires, cur_time);
> > +		if (!cputime_eq(it->incr, cputime_zero)) {
> > +			it->expires = cputime_add(it->expires, it->incr);
> > +			it->error += it->incr_error;
> > +			if (it->error >= onecputick) {
> > +				it->expires = cputime_sub(it->expires,
> > +							jiffies_to_cputime(1));
> > +				it->error -= onecputick;
> > +			}
> 
>   Yep, that's a sane solution except for jiffies_to_cputime(), which
>   can be precomputed as well.

Ok, I forgot PPC exist.

Thanks
Stanislaw
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