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

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Message-Id: <4846D647.BA47.005A.0@novell.com>
Date:	Wed, 04 Jun 2008 15:52:07 -0600
From:	"Gregory Haskins" <ghaskins@...ell.com>
To:	"Peter Zijlstra" <peterz@...radead.org>
Cc:	"Ingo Molnar" <mingo@...e.hu>,
	"Steven Rostedt" <rostedt@...dmis.org>, <tglx@...utronix.de>,
	"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, Jun 4, 2008 at  5:09 PM, in message
<1212613754.19205.25.camel@...py.programming.kicks-ass.net>, Peter Zijlstra
<peterz@...radead.org> wrote: 
> 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.

Indeed.  Here is a new version:

(Thanks, Peter!)

------

    sched: fix cpupri priocount

    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>

diff --git a/kernel/sched_cpupri.h b/kernel/sched_cpupri.h
index 16d29b9..51e6553 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 BITS_TO_LONGS(CPUPRI_NR_PRIORITIES)

 #define CPUPRI_INVALID -1
 #define CPUPRI_IDLE     0

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