linux-kernel - Re: [PATCH -mm] slub: fix cpu hotplug offline/online path

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Date:	Wed, 10 Oct 2007 21:18:54 +0900
From:	Akinobu Mita <akinobu.mita@...il.com>
To:	Christoph Lameter <clameter@....com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [PATCH -mm] slub: fix cpu hotplug offline/online path

On Tue, Oct 09, 2007 at 11:46:14AM -0700, Christoph Lameter wrote:
> On Wed, 10 Oct 2007, Akinobu Mita wrote:
> 
> > This patch removes init_alloc_cpu_cpu() from cpu hotplug notifier. But
> > call it for each possible CPUs not only online CPUs at initialization time.
> 
> Could you check if a per cpu structure has already been allocated and then 
> simply skip the call to init? Otherwise we end up with lots of per cpu 
> structures for cpus that will never show up. 
> 
> if (!get_cpu_slab(s, cpu))
> 	init_alloc_cpu_cpu( ...)
> 

I couldn't use get_cpu_slab() for that check. But I reviced the patch to do
what you said.

Subject: slub: fix cpu hotplug offline/online path

This patch fixes the problem introduced by:
http://kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.23-rc8/2.6.23-rc8-mm2/broken-out/slub-place-kmem_cache_cpu-structures-in-a-numa-aware-way.patch

I got slub BUG report when I tried to do cpu hotplug/unplug
$ while true; do
	echo 0 > /sys/devices/system/cpu/cpu1/online
	echo 1 > /sys/devices/system/cpu/cpu1/online
done

This is because init_alloc_cpu_cpu() is called every time when the CPU is
going to be onlined but init_alloc_cpu_cpu() is not intented to be called
twice or more for same CPU. Then it breaks kmem_cache_cpu_free list for
the CPU.

This patch checks if a per cpu structure has already been allocated and then
simply skip the call to init_alloc_cpu_cpu().

Cc: Christoph Lameter <clameter@....com>
Signed-off-by: Akinobu Mita <akinobu.mita@...il.com>

---
 mm/slub.c |    5 +++++
 1 file changed, 5 insertions(+)

Index: 2.6-mm/mm/slub.c
===================================================================
--- 2.6-mm.orig/mm/slub.c
+++ 2.6-mm/mm/slub.c
@@ -2033,6 +2033,11 @@ static void init_alloc_cpu_cpu(int cpu)
 {
 	int i;
 
+	if (per_cpu(kmem_cache_cpu_free, cpu)) {
+		/* Already initialized once */
+		return;
+	}
+
 	for (i = NR_KMEM_CACHE_CPU - 1; i >= 0; i--)
 		free_kmem_cache_cpu(&per_cpu(kmem_cache_cpu, cpu)[i], cpu);
 }
-
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