On 2015/8/18 8:31, David Rientjes wrote:

On Mon, 17 Aug 2015, Jiang Liu wrote:

quoted

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

quoted

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().

quoted

Signed-off-by: Jiang Liu <redacted>
---
 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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