linux-kernel - Re: [PATCH v4 12/13] fair.c: Use generic rbtree impl in fair scheduler

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Date:	Tue, 26 Jun 2012 14:15:49 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Daniel Santos <daniel.santos@...ox.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Christopher Li <sparse@...isli.org>,
	David Daney <david.daney@...ium.com>,
	David Howells <dhowells@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Ingo Molnar <mingo@...nel.org>, Joe Perches <joe@...ches.com>,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	linux-doc@...r.kernel.org, linux-sparse@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Paul Turner <pjt@...gle.com>,
	Pavel Pisa <pisa@....felk.cvut.cz>,
	Richard Weinberger <richard@....at>,
	Rob Landley <rob@...dley.net>,
	Steven Rostedt <rostedt@...dmis.org>,
	Suresh Siddha <suresh.b.siddha@...el.com>
Subject: Re: [PATCH v4 12/13] fair.c: Use generic rbtree impl in fair
 scheduler

On Fri, 2012-06-22 at 23:00 -0500, Daniel Santos wrote:
> +static inline long compare_vruntime(u64 *a, u64 *b)
> +{
> +#if __BITS_PER_LONG >= 64
> +       return (long)((s64)*a - (s64)*b);
> +#else
> +/* This is hacky, but is done to reduce instructions -- we wont use this for
> + * rbtree lookups, only inserts, and since our relationship is defined as
> + * non-unique, we only need to return positive if a > b and any other value
> + * means less than.
> + */
> +       return (long)(*a > *b);
> +#endif
> +} 

That's wrong.. suppose: a = 10, b = ULLONG_MAX - 10

In that case (s64)(a - b) = 20, however a > b is false.

And yes, vruntime wrap does happen.
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