linux-kernel - Re: [PATCH V2 2/2]: powerpc/hotplug/mm: Fix hot-add memory node assoc

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Message-Id: <895fac95-bd7a-7006-5fef-5b9f75882bcb@linux.vnet.ibm.com>
Date:   Fri, 26 May 2017 07:28:27 -0500
From:   Michael Bringmann <mwb@...ux.vnet.ibm.com>
To:     Balbir Singh <bsingharora@...il.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Reza Arbab <arbab@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Bharata B Rao <bharata@...ux.vnet.ib>,
        Shailendra Singh <shailendras@...dia.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH V2 2/2]: powerpc/hotplug/mm: Fix hot-add memory node assoc

>>  arch/powerpc/mm/numa.c |    7 -------
>>  1 file changed, 7 deletions(-)
>>
>> diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
>> index 15c2dd5..18f3038 100644
>> --- a/arch/powerpc/mm/numa.c
>> +++ b/arch/powerpc/mm/numa.c
>> @@ -907,13 +907,6 @@ void __init initmem_init(void)
>>  
>>  	memblock_dump_all();
>>  
>> -	/*
>> -	 * Reduce the possible NUMA nodes to the online NUMA nodes,
>> -	 * since we do not support node hotplug. This ensures that  we
>> -	 * lower the maximum NUMA node ID to what is actually present.
>> -	 */
>> -	nodes_and(node_possible_map, node_possible_map, node_online_map);
>> -
> 
> There is an overhead with turning this off if you have too many cgroups
> with the memory controller. I think this fix was added for a pathological
> test case. On my system I see 84 cgroups with 1 node, so the probable
> overhead is 84*255*sizeof(struct mem_cgroup_tree_per_node).
> 
> I tried some patches to reduce the overhead, but those need more overhauling
> and rework.

Is there some other way to add a node to a dynamic, running system without
crashing?  I have not encountered one as yet.

> Balbir Singh.

-- 
Michael W. Bringmann
Linux Technology Center
IBM Corporation
Tie-Line  363-5196
External: (512) 286-5196
Cell:       (512) 466-0650
mwb@...ux.vnet.ibm.com