Re: [PATCH 1/2] powerpc/nodes: Ensure enough nodes avail for operations

[Michael Bringmann <mwb@linux.vnet.ibm.com>]

See below.

On 10/16/2017 07:33 AM, Michael Ellerman wrote:
> Michael Bringmann <mwb@linux.vnet.ibm.com> writes:
> 
>> powerpc/nodes: On systems like PowerPC which allow 'hot-add' of CPU
> 
> This is a powerpc-only patch, so saying "systems like PowerPC" is
> confusing. What you should be saying is "On pseries systems".
> 
>> or memory resources, it may occur that the new resources are to be
>> inserted into nodes that were not used for these resources at bootup.
>> In the kernel, any node that is used must be defined and initialized
>> at boot.
>>
>> This patch extracts the value of the lowest domain level (number of
>> allocable resources) from the "rtas" device tree property
>> "ibm,current-associativity-domains" or the device tree property
> 
> What is current associativity domains? I've not heard of it, where is it
> documented, and what does it mean.
> 
> Why would use the "current" set vs the "max"? I thought the whole point
> was to discover the maximum possible set of nodes that could be
> hotplugged.
> 
>> "ibm,max-associativity-domains" to use as the maximum number of nodes
>> to setup as possibly available in the system.  This new setting will
>> override the instruction,
>>
>>     nodes_and(node_possible_map, node_possible_map, node_online_map);
>>
>> presently seen in the function arch/powerpc/mm/numa.c:initmem_init().
>>
>> If the property is not present at boot, no operation will be performed
>> to define or enable additional nodes.
>>
>> Signed-off-by: Michael Bringmann <mwb@linux.vnet.ibm.com>
>> ---
>>  arch/powerpc/mm/numa.c |   47 +++++++++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 47 insertions(+)
>>
>> diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
>> index ec098b3..b385cd0 100644
>> --- a/arch/powerpc/mm/numa.c
>> +++ b/arch/powerpc/mm/numa.c
>> @@ -892,6 +892,51 @@ static void __init setup_node_data(int nid, u64 start_pfn, u64 end_pfn)
>>  	NODE_DATA(nid)->node_spanned_pages = spanned_pages;
>>  }
>>  
>> +static void __init node_associativity_setup(void)
> 
> This should really be called "find_possible_nodes()" or something more
> descriptive.

Okay.
> 
>> +{
>> +	struct device_node *rtas;
>> +
>> +	rtas = of_find_node_by_path("/rtas");
>> +	if (rtas) {
> 
> If you just short-circuit that return the whole function body can be
> deintented, making it significantly more readable.
> 
> ie:
> +	rtas = of_find_node_by_path("/rtas");
> +	if (!rtas)
> +		return;

Okay.

> 
>> +		const __be32 *prop;
>> +		u32 len, entries, numnodes, i;
>> +
>> +		prop = of_get_property(rtas,
>> +					"ibm,current-associativity-domains", &len);
> 
> Please don't use of_get_property() in new code, we have much better
> accessors these days, which do better error checking and handle the
> endian conversions for you.
> 
> In this case you'd use eg:
> 
> 	u32 entries;
> 	rc = of_property_read_u32(rtas, "ibm,current-associativity-domains", &entries);

The property 'ibm,current-associativity-domains' has the same format as the property
'ibm,max-associativity-domains' i.e. it is an integer array.  The accessor of_property_read_32,
however, expects it to be an integer singleton value.  Instead, it needs:

> 
>> +		if (!prop || len < sizeof(unsigned int)) {
>> +			prop = of_get_property(rtas,
>> +					"ibm,max-associativity-domains", &len);
                        if (!prop || len < sizeof(unsigned int))
>> +				goto endit;
>> +		}
>> +
>> +		entries = of_read_number(prop++, 1);
>> +
>> +		if (len < (entries * sizeof(unsigned int)))
>> +			goto endit;
>> +
>> +		if ((0 <= min_common_depth) && (min_common_depth <= (entries-1)))
>> +			entries = min_common_depth;
>> +		else
>> +			entries -= 1;
> 			^
>                         You can't just guess that will be the right entry.
> 
> If min_common_depth is < 0 the function should have just returned
> immediately at the top.

Okay.

> 
> If min_common_depth is outside the range of the property that's a buggy
> device tree, you should print a warning and return.
> 
>> +		numnodes = of_read_number(&prop[entries], 1);
> 
> 	u32 num_nodes;
> 	rc = of_property_read_u32_index(rtas, "ibm,current-associativity-domains", min_common_depth, &num_nodes);
>> +
>> +		printk(KERN_INFO "numa: Nodes = %d (mcd = %d)\n", numnodes,
>> +			min_common_depth);
>> +
>> +		for (i = 0; i < numnodes; i++) {
>> +			if (!node_possible(i)) {
>> +				setup_node_data(i, 0, 0);
> 
> Do we actually need to setup the NODE_DATA() yet? Doing it now ensures
> it will not be allocated node local, which sucks.

Okay.

> 
>> +				node_set(i, node_possible_map);
>> +			}
>> +		}
>> +	}
>> +
>> +endit:
> 
> "out" would be the normal name.

Okay.

> 
>> +	if (rtas)
>> +		of_node_put(rtas);
>> +}
>> +
>>  void __init initmem_init(void)
>>  {
>>  	int nid, cpu;
>> @@ -911,6 +956,8 @@ void __init initmem_init(void)
>>  	 */
> 
> You need to update the comment above here which is contradicted by the
> new function you're adding.

Okay.

> 
>>  	nodes_and(node_possible_map, node_possible_map, node_online_map);
>>  
>> +	node_associativity_setup();
>> +
>>  	for_each_online_node(nid) {
>>  		unsigned long start_pfn, end_pfn;
>>  
> 
> cheers
> 
> 

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