linux-kernel - Re: sched: odd values for effective load calculations

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Message-ID: <20141219112019.GI30905@twins.programming.kicks-ass.net>
Date:	Fri, 19 Dec 2014 12:20:19 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Yuyang Du <yuyang.du@...el.com>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>,
	Andrey Ryabinin <a.ryabinin@...sung.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: sched: odd values for effective load calculations

On Fri, Dec 19, 2014 at 08:29:56AM +0800, Yuyang Du wrote:
> Subject: [PATCH] sched: Fix long and unsigned long multiplication error in effective_load
> 
> In effective_load, we have (long w * unsigned long tg->shares) / long W,
> when w is negative, it is cast to unsigned long and hence the product is
> insanely large. Fix this by casting tg->shares to long.
> 
> Reported-by: Sasha Levin <sasha.levin@...cle.com>
> Signed-off-by: Yuyang Du <yuyang.du@...el.com>
> ---
>  kernel/sched/fair.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index df2cdf7..6b99659 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4424,7 +4424,7 @@ static long effective_load(struct task_group *tg, int cpu, long wl, long wg)
>  		 * wl = S * s'_i; see (2)
>  		 */
>  		if (W > 0 && w < W)
> -			wl = (w * tg->shares) / W;
> +			wl = (w * (long)tg->shares) / W;
>  		else
>  			wl = tg->shares;


Oh, nice! thanks!
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