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Date:	Tue, 6 Oct 2015 18:01:06 -0700
From:	Jesse Gross <jesse@...ira.com>
To:	Alexander Duyck <alexander.duyck@...il.com>,
	Jarno Rajahalme <jrajahalme@...ira.com>
Cc:	Vlastimil Babka <vbabka@...e.cz>,
	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	"dev@...nvswitch.org" <dev@...nvswitch.org>,
	Pravin Shelar <pshelar@...ira.com>,
	"David S. Miller" <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm@...ck.org
Subject: Re: [ovs-dev] [PATCH] ovs: do not allocate memory from offline numa node

On Mon, Oct 5, 2015 at 1:25 PM, Alexander Duyck
<alexander.duyck@...il.com> wrote:
> On 10/05/2015 06:59 AM, Vlastimil Babka wrote:
>>
>> On 10/02/2015 12:18 PM, Konstantin Khlebnikov wrote:
>>>
>>> When openvswitch tries allocate memory from offline numa node 0:
>>> stats = kmem_cache_alloc_node(flow_stats_cache, GFP_KERNEL | __GFP_ZERO,
>>> 0)
>>> It catches VM_BUG_ON(nid < 0 || nid >= MAX_NUMNODES || !node_online(nid))
>>> [ replaced with VM_WARN_ON(!node_online(nid)) recently ] in linux/gfp.h
>>> This patch disables numa affinity in this case.
>>>
>>> Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
>>
>>
>> ...
>>
>>> diff --git a/net/openvswitch/flow_table.c b/net/openvswitch/flow_table.c
>>> index f2ea83ba4763..c7f74aab34b9 100644
>>> --- a/net/openvswitch/flow_table.c
>>> +++ b/net/openvswitch/flow_table.c
>>> @@ -93,7 +93,8 @@ struct sw_flow *ovs_flow_alloc(void)
>>>
>>>       /* Initialize the default stat node. */
>>>       stats = kmem_cache_alloc_node(flow_stats_cache,
>>> -                      GFP_KERNEL | __GFP_ZERO, 0);
>>> +                      GFP_KERNEL | __GFP_ZERO,
>>> +                      node_online(0) ? 0 : NUMA_NO_NODE);
>>
>>
>> Stupid question: can node 0 become offline between this check, and the
>> VM_WARN_ON? :) BTW what kind of system has node 0 offline?
>
>
> Another question to ask would be is it possible for node 0 to be online, but
> be a memoryless node?
>
> I would say you are better off just making this call kmem_cache_alloc.  I
> don't see anything that indicates the memory has to come from node 0, so
> adding the extra overhead doesn't provide any value.

I agree that this at least makes me wonder, though I actually have
concerns in the opposite direction - I see assumptions about this
being on node 0 in net/openvswitch/flow.c.

Jarno, since you original wrote this code, can you take a look to see
if everything still makes sense?
--
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