linux-kernel - Re: [PATCH] cpuset: fix allocating page cache/slab object on the unallowed node when memory spread is set

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Message-Id: <20090203141645.91c7ea18.akpm@linux-foundation.org>
Date:	Tue, 3 Feb 2009 14:16:45 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	miaox@...fujitsu.com
Cc:	mingo@...e.hu, menage@...gle.com, linux-kernel@...r.kernel.org,
	cl@...ux-foundation.org, nickpiggin@...oo.com.au
Subject: Re: [PATCH] cpuset: fix allocating page cache/slab object on the
 unallowed node when memory spread is set

On Tue, 03 Feb 2009 11:25:25 +0800
Miao Xie <miaox@...fujitsu.com> wrote:

> on 2009-1-28 6:42 Andrew Morton wrote:
> > On Wed, 21 Jan 2009 16:06:20 +0800
> > Miao Xie <miaox@...fujitsu.com> wrote:
> > 
> >> The task still allocated the page caches on old node after modifying its
> >> cpuset's mems when 'memory_spread_page' was set, it is caused by the old
> >> mem_allowed_list of the task, the current kernel doesn't updates it unless some
> >> function invokes cpuset_update_task_memory_state(), it is too late sometimes.
> >> We must update the mem_allowed_list of the tasks in time.
> >>
> >> Slab has the same problem.
> >>
> >> We fixes the bug by updating tasks' mem_allowed_list and spread flag after
> >> its cpuset's mems or spread flag is changed.
> >>
> >>
> >> ...
> >>
> >> --- a/kernel/kthread.c
> >> +++ b/kernel/kthread.c
> >> @@ -242,6 +242,7 @@ int kthreadd(void *unused)
> >>  	set_user_nice(tsk, KTHREAD_NICE_LEVEL);
> >>  	set_cpus_allowed_ptr(tsk, CPU_MASK_ALL_PTR);
> >>  
> >> +	current->mems_allowed = node_possible_map;
> >>  	current->flags |= PF_NOFREEZE | PF_FREEZER_NOSIG;
> > 
> > Why this change?  kthreadd() is called from rest_init(), before anyone
> > has had a chance to alter ->mems_allowed?
> 
> I found that after mems_allowed of kthreadd was not initialized applying this patch,
> every bit of it is 1, so...
> Maybe it is redundant.

I think it is redundant.  kthreadd's mems_allowed _should_ be all-ones.
Or at least, all-nodes-allowed.

I wasn't able to find out where the setting of init'smems_allowed
happens, after a bit of grepping and hunting.  It should be done within
INIT_TASK, but isn't.

Still, kthreadd is reliably parented by swapper, and there shold be no
need to alter its mems_allowed.

Similarly, what was the reason for setting current->mems_allowed in
kernel_init()?  That also should be unneeded.

Finally, I've somewhat lost track of where we are with this patch. 
Paul, do you see any other remaining issues?

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