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

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Message-ID: <alpine.LFD.2.00.0905221611380.3570@localhost.localdomain>
Date:	Fri, 22 May 2009 16:27:40 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Stanislaw Gruszka <sgruszka@...hat.com>
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, 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.

>  	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.

Thanks,

	tglx
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