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Date: Thu, 12 Jul 2012 20:27:28 +0200
From: Andrea Arcangeli <aarcange@redhat.com>
To: Rik van Riel <riel@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Hillf Danton <dhillf@gmail.com>, Dan Smith <danms@us.ibm.com>,
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Subject: Re: [PATCH 20/40] autonuma: alloc/free/init mm_autonuma
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Hi Rik,

On Sun, Jul 01, 2012 at 11:33:17AM -0400, Rik van Riel wrote:
> On 06/28/2012 08:56 AM, Andrea Arcangeli wrote:
> 
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index 0adbe09..3e5a0d9 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -527,6 +527,8 @@ static void mm_init_aio(struct mm_struct *mm)
> >
> >   static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p)
> >   {
> > +	if (unlikely(alloc_mm_autonuma(mm)))
> > +		goto out_free_mm;
> >   	atomic_set(&mm->mm_users, 1);
> >   	atomic_set(&mm->mm_count, 1);
> >   	init_rwsem(&mm->mmap_sem);
> 
> I wonder if it would be possible to defer the allocation
> of the mm_autonuma struct to knuma_scand, so short lived
> processes never have to allocate and free the mm_autonuma
> structure.
> 
> That way we only have a function call at exit time, and
> the branch inside kfree that checks for a null pointer.

It would be possible to convert them to prepare_mm/task_autonuma (the
mm side especially would be a branch once in a while) but it would
then become impossible to inherit the mm/task stats across
fork/clone. Right now the default is to reset them, but two sysfs
switches control that, and I wouldn't drop those until I've the time
to experiment how large kernel builds are affected by enabling the
stats inheritance. Right now kernel builds are unaffected because of
the default stat-resetting behavior and gcc too quick to be measured.