Re: [RFC Patch V1 17/30] mm, intel_powerclamp: Use cpu_to_mem()/numa_mem_id() to support memoryless node

[Nishanth Aravamudan]

On 11.07.2014 [15:37:34 +0800], Jiang Liu wrote:
> When CONFIG_HAVE_MEMORYLESS_NODES is enabled, cpu_to_node()/numa_node_id()
> may return a node without memory, and later cause system failure/panic
> when calling kmalloc_node() and friends with returned node id.
> So use cpu_to_mem()/numa_mem_id() instead to get the nearest node with
> memory for the/current cpu.

You used the same changelog for all of the patches, it seems. But the
interface below (kthread_create_on_node) doesn't go into kmalloc_node?

kthread_create_on_node eventually sets the value used by
tsk_fork_get_node(), which is used by alloc_task_struct_node() and
alloc_thread_info_node(). The first uses kmem_cache_alloc_node() and the
second, depending on the relative sizes of THREAD_SIZE and PAGE_SIZE
uses either alloc_kmem_pages_node() or kmem_cache_alloc_node().
kmem_cache_alloc_node() goes into the appropriate slab allocator which
on SLUB for instance, goes down into __alloc_pages_nodemask. But no
failure occurs when memoryless nodes are present, you just get memory
that is remote from the node specified? Similarly,
alloc_kmem_pages_node() calls into __alloc_pages with an appropriate
node_zonelist, which should provide for the correct fallback based upon
NUMA topology?

What system failure/panic did you see that is resolved by this patch?

> If CONFIG_HAVE_MEMORYLESS_NODES is disabled, cpu_to_mem()/numa_mem_id()
> is the same as cpu_to_node()/numa_node_id().
> 
> Signed-off-by: Jiang Liu <jiang.liu@xxxxxxxxxxxxxxx>
> ---
>  drivers/thermal/intel_powerclamp.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/thermal/intel_powerclamp.c b/drivers/thermal/intel_powerclamp.c
> index 95cb7fc20e17..9d9be8cd1b50 100644
> --- a/drivers/thermal/intel_powerclamp.c
> +++ b/drivers/thermal/intel_powerclamp.c
> @@ -531,7 +531,7 @@ static int start_power_clamp(void)
> 
>  		thread = kthread_create_on_node(clamp_thread,
>  						(void *) cpu,
> -						cpu_to_node(cpu),
> +						cpu_to_mem(cpu),

As Tejun has pointed out elsewhere, we lose context here about the
original node we were running on. That information is relevant for a few
reasons:

1) In the underlying allocator, we might not have memory *right now* to
satisfy a request, which, say, causes us to deactivate a slab
(CONFIG_SLUB). But that condition may be relieved in the future and we
want to use the correct node again then.

2) For topologies that are symmetrical around a memoryless node, we
could lose the correct fallback information when we specify a nearest
neighbor with memory.

Thanks,
Nish

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