Re: [PATCH] Silly bitmap size accounting fix

[Nick Piggin <nickpiggin@yahoo.com.au>]

Steven Rostedt wrote:
> On Sat, 13 May 2006, Nick Piggin wrote:
>
>
> > Ingo Molnar wrote:
> >
> > > * Steven Rostedt <rostedt@goodmis.org> wrote:
> > >
> > >
> > >
> > > > -#define BITMAP_SIZE ((((MAX_PRIO+1+7)/8)+sizeof(long)-1)/sizeof(long))
> > > > +#define BITMAP_SIZE ((((MAX_PRIO+7)/8)+sizeof(long)-1)/sizeof(long))
> > >
> > >
> > > Acked-by: Ingo Molnar <mingo@elte.hu>
> >
> > Really?! What about the delimiter bit set at MAX_PRIO?
>
>
>
> 		// delimiter for bitsearch
> 		__set_bit(MAX_PRIO, array->bitmap);
>
>
> Ah! I see what you mean.  New patch (add a comment).

That would have caused someone a world of pain 3 years ahead ;)

> -- Steve
>
> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

OK I guess. Does it help to also spell out exactly what's going on there?

> Index: linux-2.6.17-rc3-mm1/kernel/sched.c
> ===================================================================
> --- linux-2.6.17-rc3-mm1.orig/kernel/sched.c	2006-05-12 04:02:32.000000000 -0400
> +++ linux-2.6.17-rc3-mm1/kernel/sched.c	2006-05-13 10:09:15.000000000 -0400
> @@ -192,6 +192,10 @@ static inline unsigned int task_timeslic
>   * These are the runqueue data structures:
>   */
>
> +/*
> + * Calculate BITMAP_SIZE.
> + *  The bitmask holds MAX_PRIO bits + 1 for the delimiter.

+ * Calculation is to find the minimum number of longs that holds MAX_PRIO+1 bits:
+ *  size-in-chars = ceiling((MAX_PRIO+1) / CHAR_BITS)
+ *  size-in-longs = ceiling(size-in-chars / sizeof(long))

> + */
>  #define BITMAP_SIZE ((((MAX_PRIO+1+7)/8)+sizeof(long)-1)/sizeof(long))

--
SUSE Labs, Novell Inc.
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