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

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Message-ID: <20091020063555.GJ8550@elte.hu>
Date:	Tue, 20 Oct 2009 08:35:55 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	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


* 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.

	Ingo
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