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SubjectRe: [PATCH -mm] slub: fix cpu hotplug offline/online path
On Wed, Oct 10, 2007 at 10:39:32AM -0700, Christoph Lameter wrote:
> On Wed, 10 Oct 2007, Akinobu Mita wrote:
>
> > I couldn't use get_cpu_slab() for that check. But I reviced the patch to do
> > what you said.
>
> Why would get_cpu_slab not work?


case CPU_DEAD:
case CPU_DEAD_FROZEN:
down_read(&slub_lock);
list_for_each_entry(s, &slab_caches, list) {
struct kmem_cache_cpu *c = get_cpu_slab(s, cpu);

local_irq_save(flags);
__flush_cpu_slab(s, cpu);
local_irq_restore(flags);
free_kmem_cache_cpu(c, cpu);
s->cpu_slab[cpu] = NULL; <----------------------
}
up_read(&slub_lock);
break;

When CPU is offlined, cpu-hotplug notifier sets s->cpu_slab[cpu] = NULL.
This means get_cpu_slab() always return NULL when CPU is being onlined.
So I can't use get_cpu_slab to check whether kmem_cache_cpu_free
initalization for the CPU has already been done or not.

>
> > + if (per_cpu(kmem_cache_cpu_free, cpu)) {
> > + /* Already initialized once */
> > + return;
> > + }
> > +
>
> kmem_cache_cpu_free is not only NULL if the cpu is not up yet but it is
> also NULL if the per cpu pool of kmem_cache_cpu structures was
> exhausted.

cpu-hotplug notifier by CPU_DEAD event frees kmem_cache_cpu structures
for the CPU being offlined. So we have 100 kmem_cache cpu structures
when the CPU will be onlined again.

But I agree that it is not so trivial. It is why I added the comment
/* Already initialized once */ in previous patch.

This is another approach for the fix.
Use cpumask to check whether kmem_cache_cpu_free initalization for
the CPU has already been done or not.

---
mm/slub.c | 6 ++++++
1 file changed, 6 insertions(+)

Index: 2.6-mm/mm/slub.c
===================================================================
--- 2.6-mm.orig/mm/slub.c
+++ 2.6-mm/mm/slub.c
@@ -1959,6 +1959,7 @@ static DEFINE_PER_CPU(struct kmem_cache_
kmem_cache_cpu)[NR_KMEM_CACHE_CPU];

static DEFINE_PER_CPU(struct kmem_cache_cpu *, kmem_cache_cpu_free);
+static cpumask_t kmem_cach_cpu_free_init_once = CPU_MASK_NONE;

static struct kmem_cache_cpu *alloc_kmem_cache_cpu(struct kmem_cache *s,
int cpu, gfp_t flags)
@@ -2033,8 +2034,13 @@ static void init_alloc_cpu_cpu(int cpu)
{
int i;

+ if (cpu_isset(cpu, kmem_cach_cpu_free_init_once))
+ return;
+
for (i = NR_KMEM_CACHE_CPU - 1; i >= 0; i--)
free_kmem_cache_cpu(&per_cpu(kmem_cache_cpu, cpu)[i], cpu);
+
+ cpu_set(cpu, kmem_cach_cpu_free_init_once);
}

static void __init init_alloc_cpu(void)
-
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