Re: [Patch V3 2/9] kernel/profile.c: Replace cpu_to_mem() with cpu_to_node()

[Jiang Liu]

On 2015/8/18 8:31, David Rientjes wrote:
> On Mon, 17 Aug 2015, Jiang Liu wrote:
> 
>> Function profile_cpu_callback() allocates memory without specifying
>> __GFP_THISNODE flag, so replace cpu_to_mem() with cpu_to_node()
>> because cpu_to_mem() may cause suboptimal memory allocation if
>> there's no free memory on the node returned by cpu_to_mem().
>>
> 
> Why is cpu_to_node() better with regard to free memory and NUMA locality?
Hi David,
	Thanks for review. This is a special case pointed out by Tejun.
For the imagined topology, A<->B<->X<->C<->D, where A, B, C, D has
memory and X is memoryless.
Possible fallback lists are:
B: [ B, A, C, D]
X: [ B, C, A, D]
C: [ C, D, B, A]

cpu_to_mem(X) will either return B or C. Let's assume it returns B.
Then we will use "B: [ B, A, C, D]" to allocate memory for X, which
is not the optimal fallback list for X. And cpu_to_node(X) returns
X, and "X: [ B, C, A, D]" is the optimal fallback list for X.
Thanks!
Gerry

> 
>> It's safe to use cpu_to_mem() because build_all_zonelists() also
>> builds suitable fallback zonelist for memoryless node.
>>
> 
> Why reference that cpu_to_mem() is safe if you're changing away from it?
Sorry, it should be cpu_to_node() instead of cpu_to_mem().

> 
>> Signed-off-by: Jiang Liu <jiang.liu@xxxxxxxxxxxxxxx>
>> ---
>>  kernel/profile.c |    2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/kernel/profile.c b/kernel/profile.c
>> index a7bcd28d6e9f..d14805bdcc4c 100644
>> --- a/kernel/profile.c
>> +++ b/kernel/profile.c
>> @@ -336,7 +336,7 @@ static int profile_cpu_callback(struct notifier_block *info,
>>  	switch (action) {
>>  	case CPU_UP_PREPARE:
>>  	case CPU_UP_PREPARE_FROZEN:
>> -		node = cpu_to_mem(cpu);
>> +		node = cpu_to_node(cpu);
>>  		per_cpu(cpu_profile_flip, cpu) = 0;
>>  		if (!per_cpu(cpu_profile_hits, cpu)[1]) {
>>  			page = alloc_pages_exact_node(node,
--
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