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

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Message-ID: <4ADD6915.7000909@kernel.org>
Date:	Tue, 20 Oct 2009 16:39:01 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Ingo Molnar <mingo@...e.hu>, Jiri Kosina <jkosina@...e.cz>,
	Jeff Mahoney <jeffm@...e.com>,
	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

Eric Dumazet wrote:
> Ingo Molnar a écrit :
>  
>> 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.
>>
> 
> True, but this also save some amount of ram for some distro kernels.
> 
> If we keep this static NR_CPUS thing, we might be able to free
> the end of table, for other per_cpu users ?
> 
> if (nr_cpus_ids < NR_CPUS) {
> 	per_cpu_free_static_zone(&update_shares_data[nr_cpus_ids],
> 				 sizeof(long)*(NR_CPUS - nr_cpus_ids));
> }

That's doable but Considering that the users of NR_CPUS are pretty few
(and should be kept that way), I think it would be better to just use
dynamic allocation, which no longer incurs any major performance
difference (the only difference is constant offset vs. pointer in a
variable), for those cases.

Thanks.

-- 
tejun
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