linux-kernel - Re: PROBLEM: Off-by-one bug on sun4u SMP

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Date:	Thu, 10 Jun 2010 17:37:16 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	iconoclasmandheresy@...il.com
Cc:	linux-kernel@...r.kernel.org
Subject: Re: PROBLEM: Off-by-one bug on sun4u SMP

From: Ryan Davis <iconoclasmandheresy@...il.com>
Date: Thu, 10 Jun 2010 11:05:09 -0700

> On at least some sun4u machines, the CPU numbering starts at one not 0.
> This causes an off-by-one bug as other parts of the code assume
> zero-based numbering.
> 
> If you set max-cpus in the kernel config to the actual number of CPUs,
> the last CPU will be ignored and unused.
> This is because the CPU numbering starts at 1 but the code to check
> against max-cpus assumes a zero-based numbering.
> 
> On my computer: Sun Ultra 60 2x450mhz Ultrasparc.
> Building with max-cpus of 2 ignores the second cpu because 2 (one
> based cpu number) >= 2 (zero based max cpus)
> Rebuilding with a larger max-cpus is a workaround but non optimal.

max-cpus means "one larger than the maximum PHYSICAL cpu number", not
the number of cpus.  That's what this setting means, at least on
sparc64.
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