linux-kernel - Re: [PATCH 2/2] sched: fix cpupri priocount

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Message-Id: <1212613754.19205.25.camel@lappy.programming.kicks-ass.net>
Date:	Wed, 04 Jun 2008 23:09:14 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Gregory Haskins <ghaskins@...ell.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utonix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
Subject: Re: [PATCH 2/2] sched: fix cpupri priocount

On Wed, 2008-06-04 at 15:04 -0400, Gregory Haskins wrote:
> A rounding error was pointed out by Peter Zijlstra which would result
> in the structure holding priorities to be off by one.
> 
> Signed-off-by: Gregory Haskins <ghaskins@...ell.com>
> CC: Peter Zijlstra <peterz@...radead.org>
> ---
> 
>  kernel/sched_cpupri.h |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/sched_cpupri.h b/kernel/sched_cpupri.h
> index 16d29b9..817c55c 100644
> --- a/kernel/sched_cpupri.h
> +++ b/kernel/sched_cpupri.h
> @@ -4,7 +4,7 @@
>  #include <linux/sched.h>
>  
>  #define CPUPRI_NR_PRIORITIES 2+MAX_RT_PRIO
> -#define CPUPRI_NR_PRI_WORDS CPUPRI_NR_PRIORITIES/BITS_PER_LONG
> +#define CPUPRI_NR_PRI_WORDS (CPUPRI_NR_PRIORITIES + BITS_PER_LONG/2)/BITS_PER_LONG

(33 + 16) / 32 = 49 / 32 = 1

So its still wrong ;-)

Please use DECLARE_BITMAP and or BITS_TO_LONGS to avoid these issues.

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