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

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Date:	Tue, 20 Oct 2009 22:58:20 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Jeff Mahoney <jeffm@...e.com>, 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,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: Commit 34d76c41 causes linker errors on ia64 with NR_CPUS=4096

Of course I forgot to actually cc.  Cc'ing and quoting whole body.
Sorry.

Tejun Heo wrote:
> Ingo Molnar wrote:
>> IA64 should be fixed really - we can get past the 64K of percpu data 
>> limit anytime we add a few more pages of per-cpu data to the kernel - 
>> the scheduler just happened to be the one to cross it this time.
> 
> Christoph (cc'd hi!) was talking about re-purposing one of reserved
> generic registers (for current or something, I don't remember the
> details) for percpu base and just getting the original one via percpu
> access.  With that we should be able to lift the limitation without
> much performance penalty.
> 
>> Saying that all static percpu data must be below 64K, which will only be 
>> noticed once IA64 gets its testing act together months after it's been 
>> created is silly. If you want to enforce such a limit make it testable 
>> in a _timely_ fashion. Or fix the limit really.
> 
> Yeah, the problem probably was that it only pushes the perpcu area
> very slightly over the limit depending on configuration so it's not
> too surprising that it didn't get reported for some time.  Also, the
> linker script thing is a sanity check which is intentionally put there
> to trigger when this happens so that it can be found out clearly
> during build time.
> 
> In the long term, it will be a good idea to lift that restriction.
> That said, I really don't think we should be adding NR_CPUS sized
> array to percpu area either.  Things like that quickly become very
> scary with N*1024 cpu configurations.


-- 
tejun
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