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

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Message-Id: <20100512104817.beeee3b5.akpm@linux-foundation.org>
Date:	Wed, 12 May 2010 10:48:17 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	miaox@...fujitsu.com
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 Wed, 12 May 2010 17:00:45 +0800
Miao Xie <miaox@...fujitsu.com> wrote:

> on 2010-5-12 12:32, Andrew Morton wrote:
> > On Wed, 12 May 2010 15:20:51 +0800 Miao Xie <miaox@...fujitsu.com> wrote:
> > 
> >> @@ -985,6 +984,7 @@ repeat:
> >>  	 * for the read-side.
> >>  	 */
> >>  	while (ACCESS_ONCE(tsk->mems_allowed_change_disable)) {
> >> +		task_unlock(tsk);
> >>  		if (!task_curr(tsk))
> >>  			yield();
> >>  		goto repeat;
> > 
> > Oh, I meant to mention that.  No yield()s, please.  Their duration is
> > highly unpredictable.  Can we do something more deterministic here?
> 
> Maybe we can use wait_for_completion().

That would work.

> > 
> > Did you consider doing all this with locking?  get_mems_allowed() does
> > mutex_lock(current->lock)?
> 
> do you means using a real lock(such as: mutex) to protect mempolicy and mem_allowed?

yes.

> 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".

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

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...

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