linux-kernel - Re: [PATCH -mm] cpuset,mm: fix no node to alloc memory when changing cpuset's mems

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Date:	Thu, 13 May 2010 14:16:33 +0800
From:	Miao Xie <miaox@...fujitsu.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	David Rientjes <rientjes@...gle.com>,
	Lee Schermerhorn <lee.schermerhorn@...com>,
	Nick Piggin <npiggin@...e.de>, Paul Menage <menage@...gle.com>,
	Linux-Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH -mm] cpuset,mm: fix no node to alloc memory when changing
 cpuset's mems - fix2

on 2010-5-13 1:48, Andrew Morton wrote:
>> It may cause the performance regression, so I do my best to abstain from using a real
>> lock.
> 
> Well, the code as-is is pretty exotic with lots of open-coded tricky
> barriers - it's best to avoid inventing new primitives if possible. 
> For example, there's no lockdep support for this new "lock".

I didn't find an existing lock that could fix the problem well till now, so
I had to design this new "lock" to protect the task's mempolicy and mems_allowed.

> 
> mutex_lock() is pretty quick - basically a simgle atomic op.  How
> frequently do these operations occur?

There is another problem that I forgot to mention.
besides the performance problem, the read-side may call it in the context
in which the task can't sleep. so we can't use mutex.

> 
> The code you have at present is fairly similar to sequence locks.  I
> wonder if there's some way of (ab)using sequence locks for this. 
> seqlocks don't have lockdep support either...
> 

We can't use sequence locks here, because the read-side may read the data
in changing, but it can't put off cleaning the old bits.

Thanks
Miao

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