Re: [GIT PULL] scheduler fix

From: Ingo Molnar <mingo@elte.hu>
To: Andreas Schwab <schwab@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	linux-kernel@vger.kernel.org,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Mike Galbraith <efault@gmx.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Anton Blanchard <anton@samba.org>
Subject: Re: [GIT PULL] scheduler fix
Date: Thu, 8 Apr 2010 20:26:54 +0200	[thread overview]
Message-ID: <20100408182654.GA31578@elte.hu> (raw)
In-Reply-To: <m3ljcylymv.fsf@hase.home>

* Andreas Schwab <schwab@redhat.com> wrote:

> Linus Torvalds <torvalds@linux-foundation.org> writes:
> 
> > On Thu, 8 Apr 2010, Ingo Molnar wrote:
> >>  
> >> -	if (len < nr_cpu_ids)
> >> +	if ((len * BITS_PER_BYTE) < nr_cpu_ids)
> >>  		return -EINVAL;
> >
> > Not that it really matters, but this will now fail for no good reason if 
> > you pass it a half-gigabyte area due to overflow.
> 
> Which can easily be avoided.
> 
> 	if (len < DIV_ROUND_UP(nr_cpu_ids, BITS_PER_BYTE))

nr_cpu_ids is a signed integer which turns the DIV_ROUND_UP into a somewhat 
suboptimal instruction sequence. (havent checked it though)

So i'd suggest changing nr_cpu_ids to unsigned int [unless there's some strong 
reason to have it signed] plus doing something like:

	if (len < (nr_cpu_ids >> BITS_PER_BYTE_BITS))

ought to both result in better code and should be more readable. We'd have to 
add:

  #define BITS_PER_BYTE_BITS 3

to linux/bitops.h.

	Ingo