linux-kernel - Re: [PATCH 2/2] sched: Lower chances of cputime scaling overflow

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Date:	Thu, 7 Mar 2013 15:32:46 +0100
From:	Stanislaw Gruszka <sgruszka@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/2] sched: Lower chances of cputime scaling overflow

On Wed, Mar 06, 2013 at 05:06:55PM +0100, Frederic Weisbecker wrote:
> If this solution appears not to be enough in the end, we'll
> need to partly revert back to the behaviour prior to commit
>      0cf55e1ec08bb5a22e068309e2d8ba1180ab4239
>      ("sched, cputime: Introduce thread_group_times()")
> 
> Back then, the scaling was done on exit() time before adding the cputime
> of an exiting thread to the signal struct. And then we'll need to
> scale one-by-one the live threads cputime in thread_group_cputime(). The
> drawback may be a slightly slower code on exit time.

I do not see this part in the patch ? What I can see is just scaling
algorithm change. 

> -static cputime_t scale_stime(cputime_t stime, cputime_t rtime, cputime_t total)
> +static cputime_t scale_stime(u64 stime, u64 rtime, u64 total)
>  {
> -	u64 temp = (__force u64) rtime;
> +	u64 rem, res, scaled;
>  
> -	temp *= (__force u64) stime;
> -
> -	if (sizeof(cputime_t) == 4)
> -		temp = div_u64(temp, (__force u32) total);
> -	else
> -		temp = div64_u64(temp, (__force u64) total);
> +	if (rtime >= total) {
> +		res = div64_u64_rem(rtime, total, &rem);
> +		scaled = stime * res;
> +		scaled += div64_u64(stime * rem, total);
> +	} else {
> +		res = div64_u64_rem(total, rtime, &rem);
> +		scaled = div64_u64(stime, res);
> +		scaled -= div64_u64(scaled * rem, total);

Those calculus are not obvious. Perhaps it should be commented, how
they evolved from scaled = (rtime*stime)/total ?

> +	} else if (!total) {
>  		stime = rtime;

I would prefer stime = rtime/2 (hence utime will be rtime/2 too), but this
is not so important.

Other than that, patch looks great.

Stanislaw
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