linux-kernel - Re: Commit 34d76c41 causes linker errors on ia64 with NR

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Date:	Tue, 20 Oct 2009 09:08:16 -0400
From:	Jeff Mahoney <jeffm@...e.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Jiri Kosina <jkosina@...e.cz>,
	Peter Zijlstra <peterz@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Tony Luck <tony.luck@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>, linux-ia64@...r.kernel.org
Subject: Re: Commit 34d76c41 causes linker errors on ia64 with NR_CPUS=4096

On 10/20/2009 02:35 AM, Ingo Molnar wrote:
> 
> * Jiri Kosina <jkosina@...e.cz> wrote:
> 
>> On Tue, 20 Oct 2009, Ingo Molnar wrote:
>>
>>>> Commit 34d76c41 introduced percpu array update_shares_data, size of which 
>>>> being proportional to NR_CPUS. Unfortunately this blows up ia64 for large 
>>>> NR_CPUS configuration, as ia64 allows only 64k for .percpu section.
>>>>
>>>> Fix this by allocating this array dynamically and keep only pointer to it 
>>>> percpu.
>>>>
>>>> Signed-off-by: Jiri Kosina <jkosina@...e.cz>
>>>> --- 
>>>>  kernel/sched.c |   15 +++++++--------
>>>>  1 files changed, 7 insertions(+), 8 deletions(-)
>>>
>>> Seems like an IA64 bug to me. 
>>
>> IA64 guys actually use that as some kind of optimization for fast 
>> access to the percpu data in their pagefault handler, as far as I 
>> know.
> 
> Still looks like a bug if it causes a breakage (linker error) on IA64, 
> and if the 'fix' (i'd call it a workaround) causes a (small but nonzero) 
> performance regression on other architectures.

The linker error isn't a bug, it's enforcement. The ia64 linker script
explicitly rewinds the location pointer back to the start of
.data.percpu + 64k to start the .data section to cause the error if
.data.percpu is larger than 64k.

-Jeff

-- 
Jeff Mahoney
SUSE Labs
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