linux-kernel - Re: problems in linux-next (Was: Re: linux-next: Tree for December 1)

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Date:	Wed, 02 Dec 2009 08:24:22 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Christoph Lameter <cl@...ux-foundation.org>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	michal.simek@...alogix.com, linux-next@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: problems in linux-next (Was: Re: linux-next: Tree for December
 1)

Hello,

On 12/02/2009 01:01 AM, Ingo Molnar wrote:
>>> The problem is that on UP configurations.  Percpu memory allocator 
>>> becomes a simple wrapper around kmalloc and there's no way to 
>>> specify larger alignment when requesting memory from kmalloc.
>>
>> There is usually no point in aligning in UP. Alignment is typically 
>> done for smp configurations to limit cache line bouncing and control 
>> cache line use/
> 
> There is a natural minimum alignment for UP and it's smaller than the 
> cache-line size: machine word size. All our allocators (except bootmem) 
> align to machine word so there's no need to specify this explicitly.
> 
> Larger alignment than that just wastes memory - which waste UP systems 
> can afford the least.

This isn't usual alignment.  struct work_struct has one data fields
which is overloaded for two purposes.  Lower few bits are used to
carry flags while upper bits are used to point to sruct
cpu_workqueue_struct.  So, the number of available bits for flags are
determined by the alignment of cpu_workqueue_struct.  Memory usage for
cwqs isn't a big concern here.  Many workqueues will go away.  I think
we'll end up with less than half of what we have today while we'll
continue to have large number of works.

I'll just create alloc_cwq function which forces the alignment on UP.

Thanks.

-- 
tejun
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