linux-kernel - Re: [REGRESSION] [FIXED] [PATCH 1/3] mm/slab: use percpu allocator for cpu cache

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Date:	Thu, 2 Oct 2014 14:30:50 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Jeremiah Mahler <jmmahler@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [REGRESSION] [FIXED] [PATCH 1/3] mm/slab: use percpu allocator
 for cpu cache

On Mon, Sep 29, 2014 at 10:03:15AM -0700, Jeremiah Mahler wrote:
> Joonsoo,
> 
> On Mon, Sep 29, 2014 at 04:44:18PM +0900, Joonsoo Kim wrote:
> > On Sat, Sep 27, 2014 at 11:24:49PM -0700, Jeremiah Mahler wrote:
> > > On Thu, Aug 21, 2014 at 05:11:13PM +0900, Joonsoo Kim wrote:
> > > > Because of chicken and egg problem, initializaion of SLAB is really
> > > > complicated. We need to allocate cpu cache through SLAB to make
> > > > the kmem_cache works, but, before initialization of kmem_cache,
> > > > allocation through SLAB is impossible.
> > > > 
> > > > On the other hand, SLUB does initialization with more simple way. It
> > > > uses percpu allocator to allocate cpu cache so there is no chicken and
> > > > egg problem.
> > > > 
> > > > So, this patch try to use percpu allocator in SLAB. This simplify
> > > > initialization step in SLAB so that we could maintain SLAB code more
> > > > easily.
> > > > 
> > > > From my testing, there is no performance difference.
> > > > 
> > > > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
> > > 
> > > I just encountered a problem on a Lenovo Carbon X1 where it will
> > > suspend but won't resume.  A bisect indicated that this patch
> > > is causing the problem.
> > > 
> > > 997888488ef92da365b870247de773255227ce1f
> > > 
> > > I imagine the patch author, Joonsoo Kim, might have a better idea
> > > why this is happening than I do.  But if I can provide any information
> > > or run any tests that might be of help just let me know.
> > 
> > Hello,
> > 
> > Yeah, there is a bug. Below will fix your issue.
> > Could you test it and report the result?
> > 
> > Thanks for reporting it.
> > 
> > --------->8---------------
> > From e03ed6cc554e038b86d7b3a72b89d94e9ea808ba Mon Sep 17 00:00:00 2001
> > From: Joonsoo Kim <iamjoonsoo.kim@....com>
> > Date: Mon, 29 Sep 2014 16:30:43 +0900
> > Subject: [PATCH] mm/slab: fix cpu on/off handling
> > 
> > When cpu off, we flush all cpu cached objects to it's own slab.
> > free_block() is used for this purpose and it's role is just to flush
> > objects from array_cache to proper slab. It doesn't adjust array_cache's
> > internal fields so we should manually reset them to proper value.
> > Without this fix, we maintain free objects duplicately, one is in
> > cpu cache, and, the other one is in the slab. So system would be broken.
> > 
> > Reported-by: Jeremiah Mahler <jmmahler@...il.com>
> > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
> > ---
> >  mm/slab.c |    4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/slab.c b/mm/slab.c
> > index 1162f0e..ce289b4 100644
> > --- a/mm/slab.c
> > +++ b/mm/slab.c
> > @@ -1102,8 +1102,10 @@ static void cpuup_canceled(long cpu)
> >  
> >  		/* cpu is dead; no one can alloc from it. */
> >  		nc = per_cpu_ptr(cachep->cpu_cache, cpu);
> > -		if (nc)
> > +		if (nc) {
> >  			free_block(cachep, nc->entry, nc->avail, node, &list);
> > +			nc->avail = 0;
> > +		}
> >  
> >  		if (!cpumask_empty(mask)) {
> >  			spin_unlock_irq(&n->list_lock);
> > -- 
> > 1.7.9.5
> > 
> 
> That fixed the problem.  Thanks!
> 
> Tested-by: Jeremiah Mahler <jmmahler@...il.com>

Good!

Thanks.
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